Well, Doom's texures were designed with a certain resolution in mind (probably something
very low by today's standards). The same is true of Source (most of the earlier Source games use 512x512 pixel textures). Seeing a ported Doom texture next to a default Source texture would look rather bizarre; You'd want to work with the texture in Photoshop to get it up to the quality of other Source textures, but if you're going to spend the effort doing that, you may as well create your textures from scratch and use the Doom ones as a style reference.
All Source texture dimensions need to be multiples of 2, like 256, 512, 1024, or even 2 if that's all you need. The texture, however, doesn't need to be square. For instance, you can make a texture's dimensions 1024x2 and you'd be okay. Any values other than multiples of 2 or that are larger than 4096 won't work and you'll get an error when you try to compile it with VTex.
I don't think modeling is particularly difficult, but then again I find mapping properly extremely difficult
. If you have a solid understanding of how they work in the game and know the workings of the modeling program you use, it can be really easy to get some nice results. Modeling one of those skills where spending huge amounts of time doing it is the only way to get better at it. If you want to get into it, Google is your friend; I learned everything I know from browsing the net and asking people questions. If you'd like some help, drop me a PM some time.
No problem for the help. I've never actually completed and 'full' map before, so I feel your pain. Let me know how everything works out.
EDIT: Poo poo on you PB, you beat me to the post.