Your 145 approval isn't too difficult to get. But it is expensive.
Hmmm....I've been there twice, and I would not call it easy. It depends a great deal on which office you are allocated to (there's now a much smaller number of them) and more importantly whether your Surveyor is any good. The latter is possible, I guess, but the few good ones I know have all quit for Baines Simmons or Avisa, the two retirement shelters for ex-UK CAA staff, to both of which the UK CAA contracts out as much work as possible.
It's expensive because of the facilities you must have in place, before your application goes in, rather than the work in preparing an MOE (quite easy). You need premises, tools and people adequate to tick all the boxes and to perform the work you are approved for, as well as sufficient finance to carry it through without problems. For Line Maintenance you need a
customer before the CAA will accept an application. Catch 22? Yes.
IMHO a Part 145 approval will not be worth it for what you seem to want to do. And the doomsayers have a point; you could lose a lot of money, not least because you are totally dependent on the "Competent Authority" (ho, ho, ho) and they have their own agenda which does not include your business success.