I can see your predicament slav79. On one hand, fitting a replacement DMF would be the best policy and is what I would do if it was a paying job.. but in your case, as you say, it massively increases the repair bill to the point that the earlier (or tatty) cars are not worth repairing financially.
Say, for arguments sake the parts came to around £700, I think I'd question that on my car, which is probably only worth around £2.5k absolute maximum
It's a big job on the diesels as you know (particularly your 5-banger) but as above, I wouldn't automatically assume your DMF has had it. I think a lot is down to driver style; for example wheelspinning from a standing start etc must surely strain them to the point of collapse :/
If I were in your position, I'd fit a decent 2nd hand one and while you can't guarantee the mileage or life the 'new' part has had, you would be saving a fair bit of cash.
Is there a solid conversion available? Since these units became widespread, I've never, ever seen conclusive evidence that solid conversions cause any problems or that the DMF is absolutely necessary. Stories of broken cranks etc have always been banded about, but I've never seen proof that a solid flywheel has caused such damage...