I've been in the computer business over twenty years. Many years ago I was a Sage dealer (including the flagship Sovereign). Prior to that I was an Inspector of Taxes for 13 years (yes I'm an old git

). So I think I know something about computers and accounts.
I doubt any accountants (excluding 'one man bands') use Sage internally. They generally use something like Iris.
No matter what package you use, your accountant will probably just use it to extract data for input into their systems (and quite possibly manually). You'll save money on fees compared with dropping sheafs of paper on their desks - but they'll still do a fair bit of work to get the end result.
Accounting packages have always been stupidly expensive. They're only databases but the 'mystique' of financial control seems to let them keep on charging silly money.
Personally I'd avoid Sage like the plague. Sage excel at extracting money from people. I'll say no more to avoid being sued. Just my personal opinion of course.

If you're happy you can use Excel for your book-keeping why not concentrate on solving the stock control problem as a separate exercise?
Ah yes - the 'anything submitted by an accountant is just accepted by HMRC' myth. Undoubtedly things have changed a lot since I was an Inspector, but in my day that was totally untrue. In fact
some accountants had histories that meant their submissions would be more likely to be reviewed. If you're incorporated of course, you have no option.
I'd often see sole traders with trivial accounting requirements paying accountants fees they couldn't afford. On occasion I'd even suggest they could do it themselves (naughty me). Oh yes, they were sat opposite me despite having paid an accountant!