I have a beater 1989 740 16V wagon, christened The Noodle, with about 360,000 km on it.
The B234 engines are fine as long as they're well maintained. They are different engine block castings to the regularly-seen B23 / B230 - most notably, B234 blocks have their oil pumps driven by the timing belt and a pair of balance shafts are contained in alloy housings attached to left and right sides of the block.
Number 1 maintenance priority on any B234 is replacing the timing belt(s) at or just before the scheduled intervals. These are an interference-fit engine - so a broken belt = bent valves. Not only can old timing belts break, but the belt to the balance shaft is also known to break, and it takes the timing belt with it when that happens.
The timing belt was replaced on my car shortly after I bought it, and the belt that runs the balance shafts was removed and not replaced. The Noodle has been running fine for the last 8,000 km without it. Plus, i figured if the Volvo Penta AQ171 and 251DOHC engines never needed balance shafts to operate with their 16V heads, neither do I.
There are two types of B234 - ones with manual timing belt tensioner (early, up to mid 1990 production IIRC) while the later models have an automatic i.e. hydraulic tensioner. This also means the timing belts are different sizes. AFAIK, the later model tensioner cannot be retrofitted to the manual belt model.
It would be a good idea for you to ask the seller when the timing belts were last changed. If s/he doesn't know, presume it hasn't been done recently and plan on doing it.
It's also worth upgrading the bolt securing the oil pump pulley to the oil pump when you're changing the timing belt, as the smaller bolts in the early models have been known to break.
On a recent trip from Katoomba to Yass, The Noodle managed to do about 456 km on about 45 litres of (E10) petrol which includes stints in heavy traffic and heavier crosswinds, so it's not too bad on fuel.
Oh yeah - you might find this useful.
volvoclub.org.uk/faq/BuyingUsed7xx.html
