Carlmeek - so, looking again at your original and subsequent questions (sorry), my understanding is:
1. "I have been told by a UK instructor that after 12 years they can no longer rent or train in a (Lycoming engined Enstrom) machine".
It's true that they can't rent such a machine to you or train you in it if the engine is beyond its 12 years (+20%) life - if they own it. But if you own it - on your own - (which I assume is your intention as you talk about buying a machine) then they can train you on it whilst the engine is maintained 'on condition'. You or they cannot, however, rent it to anyone else.
2."Should I consider this as a key criteria when buying a machine?"
Yes - but the effect on value depends on what you want to do with it. If you want to rent it out or allow others to be trained on it, then you will have to have the engine 'zeroed'. However, if you only want it for personal use or to lend (not hire) it to others - and so long as the engine is good enough to be maintained on condition - then you can continue to fly it on that basis. As Fly 7 says the engine rebuild will likely cost £20k to £25k - so this should affect the price you negotiate.
Hope this answers the question.