To understand how it works under EASA and what you would have to do it's necessary to understand Part M and what the owner/operator is responsible for, summed up, I guess, as assuring the Continued Airworthiness of the aircraft, and the relationship between the owner and the person or company that does the maintenance in pursuit of that objective.
You want to be the garage, and have an appropriate approval for that, ie Part 145, but you are very unlikely to get it with the set up you have in mind, ie you with no knowledge but in charge, and a Part 66 LAE with B1 and B2 doing the work. Ever heard "Quality Management System" mentioned?
However, outside a Part 145 approval a suitably licensed and qualified Part 66 LAE can, given that a number of other boxes are ticked, issue a CRS on his/her licence alone, for a small aircraft. You are merely putting a wrapper of admin, marketing, facilities, tools and so on, around that person to enable him/her to do that. This assumes that there is a CAMO (Continuing Airworthiness Management Organisation, see Part M, especially Subpart F) looking after the aircraft if the owner isn't able to do that.
It will need an Airworthiness Review Certificate ARC every 3 years. Back to the CAMO.
You might want to consider becoming a CAMO with multiple clients, but as an FAA PPL with little experience with EASA regulations and maintenance organisations you have a steep hill to climb! And it needs the right number of competent staff.
IMHO, your starting point is not to buy an EASA-approved facility (don't forget that the approval does not necessarily continue with a change of owner), but to set up a little organisation with your friend signing off the work. But the business will depend on his/her licence and goodwill. High risk?
You can develop from there, and it's unlikely that the business you are looking at is worth a dime after a change of owner; this point is often missed with small "one-man" operations.
For interest, there's a move afoot, with the French leading the charge, to cut out huge swathes of the bureaucratic crap that EASA (and the whole ponderous, useless European Commission behind it) has generated for small non-commercial, non-complex aircraft. The UK CAA seems to be uninterested in supporting that (".....keeps us employed, old boy. More tea?") but with the French behind it things could, just, change.
If you need any help, PM me!