I agree that going from no hair to a head of hair can be dramatic and would draw unwanted attention.
some guys in that situation actually order a lower density than normal and slowly introduce a higher density untill the desired look is achieved, this usually fools many people regarding the transition.
most guys order an all lace system, French lace is a good lace for newbies as it is a bit more forgiving in handling compaired to the much more fragile Swiss lace. Believe it or not they both disapear very well despite the discrepencies in thickness between them, which isn't all that much in reality.
Full details and explanations on the base materials are on the order page and you will find more detailed explanations here on the forum.
my advice would be is to read as much as you can on here and elsewhere and build up a more detailed idea of what is actually available and ask any questions you feel the need to and i am sure there will be plenty of advice given.
Regarding density you have to match what you have on the sides and back. Your best bet is to contact John directly and he will advise accordingly.
If you go down the system route you will have to shave the thinning area, which can be traumatic for some first timers, so be prepared for the shock!