You cannot train, for the initial award of a license or a rating, in a plane unless it is maintained to Public Transport standard, or unless you (or spouse, and some other cases) own it wholly.
The above is what stopped me, at the last minute, some years ago, from buying a share in one otherwise nice plane, in which I was hoping to finish the IMCR.
The other thing is that since you are buying a share, it sounds like you will want to fly this plane long-term. In that case, the VHF avionics (VOR, LOC and radios) need to be FM Immune. More practically, the stuff really needs to work.
Have a look at the rest of the group. If the group contains some VFR-only pilots, these are likely to not want to pay to fix some piece of avionics which is relevant to IFR only. The group I refer to fell apart largely for this reason (actually there was a big turnover of shareholders) and this is quite common.
Bigendbob - I wonder how that plane gets through its Annual. If somebody does a radio check on it, they must end up doing the old trick where you placard everything "INOP", sign the release to service, and the customer then peels off the stickers
Actually I don't think this can be done legally - but I did my IMCR in planes which were worse than that!