No worries.
It involves opening the valve at the bottom of the radiator. Do this when the engine is cool, and just open it for a second and let it drain into a cup. It just gives you an idea what the coolant looks like at the bottom of the radiator and under the cap at the top.
If maintenance has been done properly, the rad is most likely fine. But, the coolant tank for the trans fluid can crack, mixing the 2 fluids, resulting in the pink milkshake syndrome. That's why inspecting all fluids before and after a test drive is important on a vehicle w this much mileage.

Lol, if I was selling a car and someone asked me if they could drain the coolant from the radiator I would probably tell them to screw off. Maybe if the guy was paying a shop to do a PPI then I'd be willing to allow that since he's probably serious. Just being realistic after having dealt with selling used cars. Plenty of characters out there that will do anything to get a quick deal.

Agreed. This question is a matter of your risk tolerance. It's old and has a bunch of miles. I've put more into my 3rd gen than I ever intended to but it's a great truck. On this whole radiator/trans thing, I suggest opening the radiator cap, when cold, and pulling the trans dip stick. Look for any evidence of a mix of the two. When I was looking, I was also reading maintenance records and if I saw a radiator replaced, coolant flushed, trans flushed in the same service, that meant that it had a pink milkshake in its past. I was not interested in buying that vehicle unless it was priced well enough that I could replace the trans.

To the OP, these things have been documented as stupidly reliable. The 3rd gen 4runner/1st tacoma platform are arguably one of the most reliable consumer vehicles ever produced. There are a few common things that I've seen lead them to the junkyard, outside of normal accidents.

1. Rust - Rust eats frames and they break. Do not buy one with bad bones.
2. Pink Milkshake - Seal between trans cooler and cooling system (in radiator) breaks, mixing coolant in the trans. The water is nearly impossible to truly flush out of the trans and leads to an early death. Depressurization of the cooling system and trans fluid in it can also lead to overheating which means head gaskets. Search adding a separate trans cooler on this forum.
3. Front lower ball joints - Search this issue. Front lower ball joints break after 150-200k miles and cause an accident leading to suspension/body damage. Replace these with OEM as a maintenance item at your desired interval (150-200k?) and you should be ok.
4. Head gaskets or hairline cracks in the heads - This is not uncommon in these vehicles at 200k+ miles. It is not an inevitable issue like it was with the 3.0. There are plenty with way more miles that have not had it happen but it does happen.

Other than those things, they're still old and high mileage. Stuff happens. There are other common but non terminal issues to look out for (rear axle seals, leaky valve cover gaskets, etc.). They're still pretty darn reliable overall. They're easy to work on and parts are abundant. When you go to resell, I think they will continue to be desirable. It will take longer to sell than a 5 y/o vehicle with an obvious market value but you should be able to sell it without too much hassle I'd think. It is a niche market and you won't be able to trade it into CarMax for a price you'd like.

I would set aside $1000 per year in an unexpected repair when dailying a 3rd gen. As braggadocious as we are about these trucks, they are aging. If it really is pristine, then try to negotiate the hell out of this. These trucks hit rock bottom in pricing nearly a decade ago and appraising them is now a blur. Being a west coast vehicle the metal will likely be very good on it.

You’re getting some good advice in these posts. The price sounds high. I sold my pristine 2000 3 years ago @239K miles for $4500. It was stock with all records, well maintained, and I replaced lots of items before they failed. Talk to the owner and look for evidence of replacing radiator, water pump, thermostat, plug wires, belts, no rust, regular fluid changes, lower ball joints, etc. If these haven’t been done they need to be, and you should factor that into your offer price.
And that yearly budget of $1000 is reasonable for preventative maintenance. You’ll need tires about every 5 years.

I have a ‘99 manual with 275,000 miles on it. I am just getting ready to do those ball joints.

Here’s a question - what is higher importance: miles or age? I am eying two 4th gen 4runners. One is a 2003 with 160,000 miles. The other is a 2009 with 256,000. Both are the same price (at a dealer).

Do the year-to-year improvements from 2003 to 2009 make up for the extra 100,000 miles? Or do you always figure less miles is less wear and tear?

I think my biggest worry with each them is potentially the transmission, but I’m not a very knowledgeable mechanic.

I think too subjective of a question to answer. 2003 was a first year model and I believe 2003-2005 had some issues. I'm not super well versed in 4th gens but I did look into them briefly. I believe the 03-05's had some issues with dashboard cracks (would annoy the hell out of me unless just a trail beater), and I think the v6's had some issues with head gaskets in those early years.

For those reasons, an 09 is preferred to an 03. Beyond that, all rubber components and everything else are 6 years older which is significant. Both are old enough that the condition they are in and climate they've been a part of are significant factors too. The 09 has 256k miles. Tons of 4runners have lasted longer than that, tons have died before. It's all a matter of risk. For reliable cars like this, I'd buy whichever is best kept and in the best condition.

Frankly, I don't think I'd be purchasing an 03, at least not until learning more about how often the dash cracks and head gasket things were. I probably wouldn't be purchasing a 2009 with 256k miles for anything more than 5k.

you have all kinds of awesome advice here op - this is great. just for me personally: i would not. thats just me. however, that does not mean you can find something excellent just follow the advice from other posters. the only reason i say no is just too many people out there look at you as a dollar sign. the vibes i get do not make me feel i am getting the sweet end of the deal.

good advice, thank you. I would love just one of your trucks!

If it's a California car with a totally rust free frame and in great shape, I could see someone asking that much for it. Several years ago, a very clean 3rd gen 4Runner from NC with around the same mileage sold for 7k according to the owner.