A 24 by 36' is not a semi-large shop. That's 864 square feet. I'd consider that a decent size shop. Compared to mine (20 by 22'-440 sq.ft.), yours is huge, but compared to seemingly most others, that's about average.
I'm about 75% in agreement with Kevin about not really needing a parts washer. Here's my reason for not going 100 percent. I think a parts washer qualifies as a "seldom used, but near must-have when I need to use it" item. A cherry picker, an engine stand, 3 1/2 ton rapid lift floor jack all rate in that category, IMHO. Granted, you can't rent a parts washer like you can rent or borrow any of the others, but boy, when you need to use it, it's nice to know it's there.
Now I'm not going to tell you that you absolutely have to have it either. I did just fine without mine for a good, long time (partly because I had an in with a buddy who had one I could use any time...). As with most any larger shop tool, you can make due without owning it if you plan ahead. I still don't own a drill press

, and I don't own a decent bench grinder either. You just learn to live without.
I've been working on writing up a sort of garage/shop vs tool-need listing to maybe contribute to the knowledge base for some time now, and I figure I'll be posting it in a month or so. Along with that listing, I'm including little tips about better shop layout that I've picked up on though various sources. Should be good reading once I'm done with it.
In a while, Chet.