Nice enlarger. Check out KHB this enlarger has been marketed under various names. The do sell a manual reprint for it. I have a manual for 4550XL online, it should be very similar if you want to have a look.

Most VC paper will have filter setups on the instruction sheets. Check out

I've never used glass carriers. Lots of opinions around. I don't think I'd bother for 35mm might try sometime on larger formats.

I'd buy the best lens I could find. Lots of good ones out there. 50mm is normal for 35mm, most 80mm will cover 6x7. The 80 is also useful for 35mm for smaller enlargements (under 8x10) it gives you more working room. Same idea with a 135 for 6x7. So don't walk away from a longer lens if you can get a good buy. I usually use an 80mm lens with 35mm when making postcards!

There are a few lens boards available, most 50 & 80 lens will fit a standard 39mm board.

How much depends. The 4550XL I bought for $200, it had a VC filter module, 3 El-Nokkor lens, 4 neg carriers, etc. I noticed one for sale locally for $900 with a bunch of stuff - still for sale I think. So price will vary depending on many factors, most important seems to be be how bad the person selling wants to get rid of it and if you can convince them it's going to a good home.