I found this 1996 Jeep Cherokee for a good price, it's got the 4.0 sixer/slushbox in it, and I'm wondering, assuming it's had good maitenence, and isn't beaten on, how many miles I should expect it to last me before rebuild time?

Also- is there anything I should know in regards to getting more power out of it? (head swaps, etc.)
p.s. yes I know it's rather vague, but I'm asking for general numbers or for guys who've had experience with these mills.