Great to see more of us getting into film. I've had a 4x5 for years but could only use part of the negative in my old 6x4cm enlarger because of the high cost of 4x5 enlargers. I tried building one but getting even illumination was a problem. I finally got lucky and got hold of one from a colour lab closing (of all places) in Adelaide. For $800 they gave me a Durst 5x4 enlarger, Wallner Analyzer and Timer and 150mm lens. Works great. This leads to a couple of things. First, pro labs are still closing down, and need to sell their gear. Collect from them if you can because you need to know what does what! (manuals are mostly lost by now). Second, you'll need more than an enlarger. You need a lens (say 150mm) and this could be an extra. My guy threw in the lens free because it had to go somewhere. Third, enlargers are either diffusion or condensor, colour or mono. Diffusion is cheaper; colour has correction filters that are ideal for variable contrast papers. The choice you make may influence both price and what you can do. All that having been said, I think with a bit of common sense you could build your own diffusion enlarger by copying the Durst design, which is pretty simple, and using Ilford's variable contrast filters for filtration. Getting everything square would be the tricky part, as would be the need for cooling (Durst uses a 200W dichroic bulb which gets pretty hot) without introducing vibration. I'm happy to post the basic Durst measurements if you're interested. Good luck.
Ian.