Keep in mind that m102404 is referring to no prohibited parts/items. Prohibited parts/items (ie. whole guns) you should always buy locally.
As for rentals they can be as cheap as $20 or up (or a case of beer or w/e).
A day of play should cost you about $60-80. Say $20 for field entrance, $20-30 for rental gun, mags and whatever, $10 for a bag of BB's, $5 for gas (if car pooling), $10-15 for timmies and lunch (picked up from your local grocery store on the way to the field or w/e). Good idea to pack your own lunch though, make it something simple and healthy like an apple and a sandwich, a cheese toastie, or a cornish pasty (my favourite, since you can hold it by the edge and even if your hands are dirty it's no big deal)).
As for upgrade parts, it's really your choice, internal upgrade parts aren't prohibited for import only things like receivers and such. I prefer whichever offers the better blend of being cheaper and easier (ie. $50 and wait 6-8 weeks for delivery and get dinged $15 for duties or pay $80 and get it in your hands the same day or have it mailed to your doorstep in a week or so).
Also, don't worry about upgrading your gun at all when starting out. In fact I haven't touched my gearbox yet and it's been left the same as the day I got it (I however bought my gun used and the previous owner was a gundoc). Play with it for a bit and figure out what you want from the gun first. As well, "upgrade the player not the gun". Sure there's a minimum level of performance that you should have but don't spend money on unneeded upgrades when stock is alright. That said.... I've been taken out with a dollar store 100FPS springer pistol before but only because it was a small field and a FIBUA kind of scenario.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE