How about the Mcintosh C50 preamp

Saw a C50 preamp for $3500 in excellent condition. 

Not sure if this is a good deal,  also want to hear Mcintosh fan's opinion on this preamp. 

Currently I have a Audio Research L15 tube preamp in the system,  but I am going to add a Mcintosh MC402 power amp to replace my MC2105,  thought this C50 pre is a good company to the MC402 by nature..

The price you mention for the C50 is about what the C48 is going for now. I had a C48 and liked it a lot, but had to sell it for financial reasons. So if you like it, I'd go for it. It should match up well with the 402.
I use a McIntosh C50 in this system:

Very happy with the C50, looks and sounds great plus legendary McIntosh build quality and reliability.