I'm not sure exactly where you'd find it or what to search for but pretty much everyone on the forums is going to recommend against it.
If you like the game, do yourself a HUGE favour...pick up or download a copy of the GOTY edition. If you buy more than a couple of those unlocks, you'll save yourself money and probably more importantly, LOADS of aggravation.
If you have the online pass already and it's only activated on one system (even though you no longer have that one), then I think getting a cheap GOTY edition is still the way to go.
FTP with your desired "upgrades" maycheaper if you don't want too much but based on some of the related threads on this topic, would probably prove to be a lot more frustrating.
I don't think ND intended too many people to leverage the FTP version heavily...it seems like the unlocks and whatnot were not implemented well. I think FTP was intended only as a teaser to get people to buy the full game...in that they may have succeeded!
don't buy the upgrades... ever...
they are overpriced, just buy a new (or used) game of the year copy, it's cheaper and then you have access to everything, the online pass is still on your account, so you can simlpy reactivate it on your new ps3 (as long as you never have shared your account with someone else that is...)
Since the online pass is on your account already just buy a used Copy of the GOTY or vanilla game DLC Maps are free anyways, unless you want skins & co op add_ons go with used GOTY. $20 for a full MP unlock is a rip off.