to keep highway speeds capable, you're gonna need a totally different motor. my 87 sammy just wont go over 73. I dont think any of them will.
for tires, I'd consider a 235/75/15 in the model of your choice, which requires a lift, and/or some body modifications to fit, but you gain a lot of aggressive offroad tires that are capable of lots.
Welcome btw, this place is VERY knowledgeable really in everything imaginable pertaining to the SJ, Samurai, trackick, etc.
MY samurai is very fun just to drive, and is my daily as well. handling is best described as being akin to a gocart, if everythin is tight in the steering system, it just GOES whereever you want it to. its a good vehicle though. mine makes me smile all the time.