I have done several refurbs, on the aircraft I own, and a number for maintenance organizations. Your best starting point is to look at, and speak to the owners of aircraft done by the company you're considering. There can be a wide band of interpretation as the what the finished job should look like. Bear in mind that most aircraft left the factory with much more casual interior furnishing quality than would be accepted on even the junkiest new car these days!
Technically, it requires design approval (and STC) to change interior furnishings, but you will find a wide range of what shops are willing to sign out as repairs. Agree to all of this before hand, and in the quoting stages.
Any non metallic materials used are subject to demonstration of flammability compliance. This is a fussy task, as a simple statement from the maker, technically does not cut it. Acceptable test reports might be called upon - depending upon who is asking.
Do not cheap out. You're not going to all that effort, 'cause you think that you're going to resell the plane the following year (right?). Pay to have it done well. Leather for the seats, wool for the headliner, very good quality carpet*, and kydex panels where you can get them.
If paining inside is required, consider "splatter" paint, or texture paint, it tends to look better. Have it properly applied though - I spray canned one of my planes, and it is peeling
. If they're painting, mask everything else, splatter paint looks horrible on the face of your GPS!
For carpet, I recommend many smaller pieces, all edge bound, and velcro'd in, so you can easily get them out for cleaning, or replacement. Buy the "rest of the roll" - you'll need it one day. Or, and I prefer, use "coin matting". It is heavy and expensive, but it looks great, cleans easily, and lasts forever.
That's a start for you, this is a big topic!