Hi Alexander,
I am actually happy to see that you posted here. I was looking at a Contax 645 system, and I found your post on photo.net with the link to your website. Your photos are superb! I also read your "about me" section and found it very coincidental. I am American, but I studied Russian and Japanese in university, and actually spent some time at Yaroslavl' State University (though I did not make it to Rybinsk) and 10 months at DVGU in Vladivostok. And then I got my masters in Southern California. So we seem to have traveled to all the same places and then both decided on photography!
In any case, on to your question. I use an M7 and 50/1.4 ASPH and I like it very much. It is a superb lens, but the 50/1.4 pre-asph is probably more like your 80/2 in the Contax 645. I don't think the 50/1.4 ASPH has bad bokeh, just not as nice perhaps. Here are a few sample shots:
Let me just repeat, all those above were with the 50/1.4 ASPH, not the pre-asph. I think the pre-asph is a great lens and is a bit softer in the way that it draws an image. It is probably also closer in character to the excellent 35/2 in the Hexar, so if you are happy with it, I would go for it. The price you quoted could be ok if everything is in superb condition. Prices are probably higher in Moscow than they are in the US, but here you can get a "used like new" (i.e. unopened, but without US warranty) M7 for 2600 dollars from Popflash. And that is indistinguishable from new, only without the full USA warranty, but with the Popflash 1 year warranty. Tony Rose (the owner) is absolutely fantastic, and very reliable. A 50/1.4 pre-asph can be had for less than 1000 dollars. In fact, the last ones were sold for 1000 dollars NEW, so the used ones are probably around 800-1000 these days. If you were in the US, I might say the price was a little high, but if it is local, then it might be the easiest option, especially if you can get a guarantee from the shop, and you can inspect it and so on.
Well good luck on the purchase and keep up the excellent photography!