Not bad at all... would need to know its history though, especially at 70k so just on the cusp of the cambelt service
If I go for a whole new engine I'll probably get something bigger tbh, doesnt seem worth just replacing it with the same.
Any used engine needs a full belt service (done properly), the reason yours has failed is presumably because the idlers weren't changed, as they aren't on the service schedule. Up until recently the cambelt kit was just a belt & tensioner, although there is now a belt kit which comes with the idlers too. Idler failure is the usual cause of belt failure on V6s where the belt has actually been changed, along with the tensioner. We just bought a minter the other week on a W reg, same thing happened to that, belt changed 30k ago in 2008, no idlers and so it inevitably wore through and failed. Shame to break it up, but its not worth putting an engine in & warrantying it for what its worth to sell.
age, history, mileage, price?
Hows the swap when you go to a bigger lump? I take it you'll need the ecu but what about ancilliaries like ac pumps etc?
Much wiring involved too? Wishful thinking that its as easy as a bmw swap, just unplug the engine loom from the body one and plug the new one in?
If you get a 3.0 from a GTV its an easy enough swap. Use 3.0 injectors, 2.5 ECU, 2.5 MAF, 2.5 oil filter housing & heat exchanger, 3.0 chrome runners & base plates, 2.5 exhaust manifolds, 2.5 flywheel & 3.2 clutch and it will run & drive brilliantly. A remap is a bonus, not essential. Unfortunately the 3.0 ECU will not fit the 2.5 car's wiring loom, but the 2.5 ECU will run the 3.0 engine just fine. Use 2.5 ancillaries (as you know they all work) rather than the ancillaries on any engine you buy.
If you get the 3.0 from a 166, there are a few other bits to change, some engine mounts and things like that, but not a lot more work.
3.0 engine & 2.5 gearbox = fantastic acceleration and great fun. Won't even be worse on fuel driven normally.
Make sure whatever engine you buy is a CF2 (assuming your 156 V6 is also a CF2) and all the sensors & breather pipes will be in the same place. Avoid the 3.0 24v from the phase 1 GTV V6 and the 164 as they are more complicated to fit, 164 has the thermostat in the wrong place, the cambelt arrangement is different and the cambelt tensioner is out of production, and the GTV V6 phase 1 has exhaust gas recirculation which you don't want.
Best of luck - sorry to hear of your trouble and if there's anything I can do for you give me a shout. I have a pair of good 2nd hand heads if you want to go that route, or a 70k 2.5 engine (cambelt history unknown), and a 79k 3.0 V6 engine (again, no cambelt history).