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124Spider-- I've got a '76 1600!-- I'd say you should definitely get a PM for the bike. Your level of interest and the fact you already use power indoors suggest to me that you could put on-road power to good use in monitoring your fitness and training, as well as engaging your interest in health and cycling.
With regards to pedal meters, have you looked at Powertap P1s? I have clubmates who use both those and Vectors, and the consensus is Powertap P1 are much easier to setup and use.
You are correct that getting into a new PM/computer setup would be about $1.4k for most scenarios (e.g. P1 + Lezyne EnhancedSuperGPS), but as mentioned upthread, you could get into used and/or hub meters for much less if the cost is really going to be a stopper.
To PepeM's comments about "considerably different" power readings, I'm not sure what they meant by that, but I think it's worth saying that PMs are pretty accurate, and while variance between units is normal, there's greater variance in human performance comparing indoor and outdoor rides. It's just easier to make bigger power outdoors for most people, for a variety of reasons. Perceived effort is very hard to compare between indoor and outdoor efforts, which I think is the main issue.
With regards to training numbers indoor and out, I don't have any insight there. I do my power training almost exclusively indoors, and then just use "what I got" outside, relying on how I feel to guide my efforts. I race rarely, but even then don't watch my power because I know I'm capable of more than I think. Most of my outdoor rides with the club are for skills building, endurance, stamina, and general training; I don't do the kinds of specific efforts outside which I do during training sessions indoors, so only indoor numbers guide my training efforts.