I AM NOT A PRO DETAILER, I DONT EVEN DETAIL FOR PROFIT:\
My 2 cents is this, depending on where you see your clientele trending, I would go that direction. If your clients are bringing you higher end cars for detailing, maybe it's worth going into PPF, if not perhaps wait it out til you build a bigger clientele and then proceed. Not that all cars that get PPF are high end but most regular car owners probably won't go more than a clear bra to the front end.
If your clients are bringing you high end luxury cars every day then perhaps it's worth looking into expanding your service offerings. Logically a customer who buys a brand new $25K car isn't going to have the money to shell out another $5K for a PPF wrap. (I say $5K based on what I've seen some companies charge, again **NOT SURE**).
I think if you're interested in expanding, start small: start doing partial wraps, i.e. mirrors, high risk areas (for paint chips), then depending on the business move further.
Although training won't be cheap and neither will materials (as you stated) so I'd make sure you're in the market for it before you jump in...although I've seen your work and it's fantastic so I'm sure you guys would be solid