My son and I did mine, he's a mechanic so he did most of it. It's not hard, but it is a LOT of work. Print out the pages of the service manual and follow along and double check things. I drove mine for weeks after it first started with no issue. I have not heard of anyone having anything other than a very small leak. Just watch your coolant and don't let it overheat, if it does definitely check your heads.
I had about 900 in it including machine shop work, gaskets, plugs, head bolts, water pump, see in the links below for a breakdown. They say its like a 20 hour job, I think we were under that even with having to take the heads apart after since we didn't measure the shims before, that step isn't mentioned in the manual. If you have questions I will try to answer.
Everywhere I read said get the engine overhaul gasket set, which I did. We replaced all the gaskets and seals we could, valve seals, injector o-rings, exhaust, intake, front seal, crank seal, etc, etc, etc. I had the heads checked and trued up at a quality machine shop. We replaced the water pump, thermostat and housing, one questionable injector, plugs and crank bolt. We broke a couple exhaust manifold studs, you can only buy them at Toyota and they are EXPENSIVE, I found some that worked from autozone. We broke one on the intake I believe. On the intake there are some torx bolts, don't remove those. I used bolts instead of studs on the intake.
Measure your valve shims before you take the heads off and make sure you keep track of where they all go.
Here are a couple of posts I made that you might find helpful,
one,
valve adjustment, not that hard or expensive,
Replacing bearings in pulleys - V6.