I know this thread is ancient but I own a hardbody and have gained a TON of knowledge about them so here it goes.
What years did they make the hardbody: 86-97
What years should I avoid: 96+ Because they are ODBII
Easy to fix: Very
Common problems: T Chain, Rust, Seized Lifters, blown rods & bent valves (if someone beat it) (dont be mislead the T-chain is easy and cheap 200 bucks DIY)
What to look out for: timing chain & excessive rust (comes from inside out, appears as small bubbles near fenders and doors under the paint, FIX ASAP or holes will be there in no time)
4 cylinder or 6 cylinder: My opinion both are great but I love the KA(4)
gas mileage? 28-30 (My 96 4cyl 5-spd Rwd)
how many miles on the odometer is too many: 200+
forums for the hardbody:
how cheap can I get one? -1000 to 5,000 depending on miles, work, & condition
reliable as old toyotas: Yes but not an old Hilux
is the old Datsun 720 as reliable: depends how good of shape they are in but the old motors are a bit more work, solid lifters and carbs
Bit more info:
They share the same motor as the 1989 S-13 or 240sx here in the states so the intake can be swapped on the -96's with relative ease. All the Ka24e performance goodies for the 240sx bolt right up to the D21 hardbody as long as it is before 1996 and the KA24 engine. The v6 is a VG30 which is almost the same as the 80's 300zx's. Some people have swapped the 80's 300zx Turbo motors in as well they bolt right up to the V6 equipped Hardbodies. All sorts of swaps have been accomplised with this truck, KA24DE, SR20's, RB20's, RB26's, u name it it has probably lived inside it.
They are becoming more and more rare so if you like em, snap one up and restore it before they all end up as iron dust in the wind.