I've seen lots of houses that use standard kitchen cabinets as a master bath vanity -- around here at least it's considered an upgrade. A 36" height is not uncomfortably tall for anyone but small children (which is why they're only used in the master bath). My wife is 5'2" and loved the 36" double vanity in our old house.
Some of those minimum clearances are defined in building codes and will vary from place to place. In Ohio we use the International Plumbing Code (IPC) and it does define some of these minimums and local municipalities might add additional regulations. But it sounds like you may not be as concerned with the minimum specifications, what you really want to know is what "feels comfortable" and looks/feels good. Unfortunately a question like that is tough to answer.
Luckily people have been dealing with small bathrooms for ages so there are a number of strategies you can employ and there are some specialty fixtures that can help, like narrower bathtubs (28” instead of the standard 30”), shallower vanities/sinks, shallower toilets (10” rough-in instead of the standard 12”), etc.
If you can find a copy of the codes your area uses that can be a good resources if you have the patience to read it. I would recommend an electronic version so you can search for terms.
In the IPC the relevant sections I found are here:
Section 417.4 Shower compartments
All shower compartments shall have a minimum of 900 square inches of
interior cross-sectional area. Shower compartments shall not be less
than 30 inches in minimum dimension measured from the finished
interior dimension of the compartment, exclusive of fixture valves,
showerheads, soap dishes, and.safety grab bars or rails.
417.4.2 Access. The shower compartment access and egress opening shall have a minimum clear and unobstructed finished width of 22 inches.
Section 405.3 Setting
405.3.1 Water closets, urinals, lavatories and bidets. A water closet, urinal, lavatory or bidet shall not be set closer than 15 inches from
its center to any side wall, partition, vanity or other obstruction,
or closer than 30 inches center-to-center between adjacent
fixtures. There shall be at least a 21-inch clearance in front of the
water closet, urinal, lavatory or bidet to any wall, fixture or door.
Water closet compartments shall not be less than 30 inches wide and 60
inches deep.
Best Answer
If you're a little computer saavy, give Google Sketchup a try. It's a free basic (but powerful) 3-D design program. What's nice about it is that it's connecting to the Google object library where you can download all sorts of already pre-built objects that people have already made.
http://sketchup.google.com/