I heven't heard the Lambda Pro or Signature, just the original one, 303, 404 and Nova sig. The old one is smoother and fuller to me, with a less sharply defined top end... some call it the Stax etch, as in it's too sharply presented.
Buying second hand is always a good thing, especially with Stax as for the most part they are very reliable, however, the Lambdas are famous for never dying. The blasted things live forever. When the end comes, the cockroackes will be sat there eating twinkies and listening to the Lambdas
As to the SRD's, the MkII is more valuable to many of us running both normal and Pro sets, there are probably less of them in circulation too which pushes the price up. However, as you've found yourself, the Normal bias runs them just fine so you don't need to rush into a Pro box. The transformes are the same, only the bias circuit is different.
Whichever one you find will be fine... be prepared for a battle though, there are quite a few Stax fans who will want it as well
I bought mine through a Tokyo classified ad and a lot of negotiation with the seller.
Good luck.