MF backs will most likely not fail...ever....no moving parts, so they won't wear out....there might be some repairs of some kind, but other then that these thing will last forever....
if oyu are concerned about "outdated", you should look for a back that fits your workflow...e.g.: for editorial work i would say it needs to be able to do about 45 shots/min, at least up to 400asa, resolution is less of an issue, superwide probably as well, tethered support should be solid for cover shots or studio set-ups.....
the perfect backs for this would be the P20 or P21, both will produce files that are far superiour to DSLR files, the P20 is 8000 new and don't forget that once you are with one back manufacturer you can always trade up.....i can tell you that i had the P20 and got more trade in then what i paid for it when i got my P30....
i would also strongly recommend finding a local dealer to start out, they will be happy to hold your hand and not only get you a back to start out, but also help you trade up and so on...it is also nice to have someone to call right away and maybe get a back-up within hours in case somehting does go wrong (we all know how that goes....)
once oyu have a back that fits your workflow, (shoots faster then your flashes recycle anyway,....) it won't get outdated...there will be better ones coming along, but you don't have to step up.....
if you shoot editorial you should be able to pass a rental fee on to your clients anyway and with that you will actually make money by renting out your back within 1-2 years....all the time shooting for free for yourself....and then trading it in (or selling it on ebay)....