If you have been stringing at a pro shop then you may be familiar with the type strings that many hit with in your area.
If you are not sure as to what strings your customers may ask for, then in beginning I would not keep a great inventory until you know what strings will be moving.So you may wish to get single sets in beginning so you will not have reels of string that does not sell.
Many hitters are familiar with brand recognition, like Prince, Luxilon, Babolat etc. Until you build up a customer base and reputation you may want to stick with brand names until your customers know you well enough , where you can then get some other brands that preform well.
PSGD has been #1 selling string in pro shops for a good # of years, but you should know that stringing in a pro shop.
As far as what to charge, you can base it on your area as to other stringers service.
I base mine on what my time is worth to me. Only you can answer that question.I rather string fewer racquets and get a fair price for services than string a good # cheaper, as I only have so much free time.This is your decision and yours only.
As far as machine, since you have experience stringing and know a quality machine of a pro shop as well as the drop weight you now use, you should know the benefits of a high end machine or a cheaper one, but to run a business, I feel its best to get as much machine as you can afford, as compared to someone who never strung before and does not know how long they will be stringing for.