Is it easy to do? Nobody will be able to truly answer that question without seeing your files.
Theoretically, yes, it is easy. Convert all of your files, update the file references to make sure everything is tied together. Host everything on FMS14 (no point in stopping at 12 if you're converting). Then carry over the sales members.
There really is probably no good reason to combine everything into one file. FMS12+ works pretty much the same in multiple files linked together. The big variable here is your sales team users.
I would recommend finding a new consultant to assist you, at least mentor the transition. The consultant search is a good place to start, if you still have trouble though I'm sure there are dozens of highly experienced people reading this thread that can also offer their assistance (myself included).
Remember too that FMP has come a long, long way since v7. Now that there's some clarity (hopefully) in the business's needs, you might find that it's better to just start fresh and import your existing data.
This might sound daunting but if you find yourself the right consultant, it may be a much easier process than you think. Good luck!