My darkroom is a partly converted bathroom, and only slightly bigger (I think) than the space you've allocated. I have a 4x5 enlarger, and work with everything from 35mm films up to 8x10.
My sink is a single-bowl stainless kitchen sink placed where the original bathroom sink and cabinet had been. My counters are built over the toilet and bathtub which are still in place.
For most print processing I use a Nova quad slot processor. They are pricey, but very, very handy for a small space darkroom, if you can fit one in your budget, I highly recommend them, it might help reduce the costs in other areas.
The Nova is on a counter and I work around it with trays on the counter if I do stuff that requires different processes than the Nova is set up for, lith, for example.
If you use a large sink, ideally you will want one that can fit 3 or 4 trays that are large enough for the biggest print you'd like to make. In the space you've allocated it might be difficult to go any larger than 11x14 inches. The alternative is a large counter space with a small sink somewhere, but spills become more of a problem when you work with trays on a counter. If you build your sink, you can make it exactly the size you need, which would be my recommendation.
The main thing I've found with working in a small darkroom is that you must stay organized, allocate a place for everything, and make sure it stays there, except when you're actively using it.
As much as I'd like the comparative luxury of the large darkrooms you see in the DR pictures thread, I've found I like working in a small space, it's very efficient. The only limitation I'd like to change is the ability to make 16x20 or larger prints. However, realistically it isn't very much of a compromise, and I have a darkroom available to me where I can make large prints if I need/want to.