Thats a lot of question so lets start with mileage on engines. New engines when taken care of last a long time. So I would guess you will get another 100K out of the engine. Rebuild downtown is $100/hr to remove and install new. You doing it $2K for a new rebuilt. Price of the rig is area depoendent and you have to be able to judge the condition it is in hard to tell from where I am at so yes blue book is a great guide for a starting point.
Myself I usually buy vehicles with at least 100K on the speedo since I am fortunate and can do all the work myself except welding and fine body work. I wouldn't worry it still is your judgement call and your money so you may want to ask if you can take it and have it lookked at. Pay a mechanic to do a quick inspection and see if it really is a good deal.
89 2.5 TBI, 4" RC lift, 1" Booms, SYE, Adams Driveshafts front and rear, Ford 8.8 with 4:88s, BFG 33X12.50X15, and a ton of other stuff.
AF Jeep Club Tail Number 184