Looks like you've locked in on one of the best bass designs on the planet - the Thunderbird.
That said, if you want to spend your money wisely in that arena there are three excellent choices. Any of the three are great and will serve you well. Probably the best logic to use is to buy the the one you can best afford.
The choices I'd recommend in order of best deal first...
1) a used Greco thunderbird for $1000 or less. Depending on where you live these can be easy or hard to come across. So look before you buy something else but don't wait long if you don't find one or you might miss out on better deals in the used TBird VP market. These are absolute monsters! They'll bury just about any other bass out there, massive volume levels, massive aggression and all the grit and grind you'd ever want.
2) a used TBird VP for $700 or less. If you can find one that comes with a case at that price jump on it, it's a hard deal to beat. These are a bit more agressive sounding than a CP and are capable of dialing in some very nice grit and grind.
3) a used CP TBird for $500 or less. Still an excellent sounding and built bird. It will sound much like its more expensive big brother Gibson Thunderbird, which is to say, excellent. A bit darker and smoother sounding than a VP
Note : if you have to buy new, your only choice of the bunch is the VP.