I have 1995 T100 with 275k miles and the engine on it's last leg. Is it worth replacing the engine due to the mileage on the transmission and the rest of the truck??

 

is it worth it to you? Do you like the truck? Can you afford a new vehicle? what else is wrong with the T100?

 

In my considered, yet totally unprofessional opinion, the answer would have to be a most resounding YES!!!
What's wrong with your motor? Most of them, unless badly damaged somehow, usually can be repaired for much less than replacing them or the truck would cost. Just trying to get a feel for things over all. I would still say yes, though. look at my two I've had the p/u since 89, and I bought it with 30k on it. I love my trucks, and can't imagine driving anything else, although if I ever DO sell, I'll get another Toyota!
Your tranny and everything else should be in good shape. May need a good flushing, but other than that...These trucks are really amazing. If you give them a little regular care and maintennance, they'll last forever...