300k is pretty dang good for a clutch, but as said it's definitely time. When clutches start to go they die in a hurry because load gets spread unevenly.
Dropping a trans isn't necessarily hard, just very tedious work. Set a weekend aside, be patient and take your time and you should be fine.
That said, it would be wise to replace as much as you can while you're there if you put in all that work to take it out. I would put this at a much higher priority than valve covers and timing belt.
I'm gonna bet a shop will quote a minimum of $1000, depending on hourly rates and how fast they work. It would go a lot faster on a lift like what they have, but some mechanics have been known to milk stuff like this for extra cash. I wouldn't take it to Toyota unless you want to be quoted the price of a new car - when I got my trans fluid flushed at my local dealership they tried to sell me new valve cover gaskets for $800.