The real questions are:
1. How desirable are the bikes and what's the resale value? What's your then cost of rebuild to investment ratio?
No question, with any bike, usually you will put more in than you can get out of it but the ratio of what you can get back and how easy it would be to sell that bike will be different. i.e. if you put $7000 into a 1988 YZ250 vs. $7000 into a 1981 Maico 490. The Maico will have larger resale value and sell faster so the loss ratio will be less. The YZ250 might sell for $1500, the Maico may sell for $5000.
2. Do YOU like the bike enough that it would be worth it to YOU to put the time and effort into the bike. This is where even if you got the 1988 YZ250, if you love that damn bike so much you put it into your bed between you and your wife that putting $7000 into it is a don't care, then go for it. However, if you're just "eh, its a bike" then I'm not sure how happy you would be when you have to open your wallete to spend money on it. Then you add it all up in the end you could have just saved it all and bought a better bike.
The second question is personal preference and no one here can help you there. It wouldn't be worth it to me as I'm not a big fan of KDX's as I understand they are quite doggish.
However you mention you ride with fast guys and 125's are quite fast, they don't have the power, but they rev to 12k and can be geared to 65 and even 75 MPH, actually can be faster (top speed not in accelleration) than a 250 since 250's don't rev that high so the same gearing has 250's with less top speed. You have to keep them at peak revs all the time and they give you more of a work out. The bigger the bike the lazier you can be.
I don't ride fast unless I'm riding with fast guys, so the big bore allow me to be lazy however they also can accellerate in the blink of an eye and I always love to blip the throttle!