Why use pressure treated lumber? Sawdust in the shop
is enough of a challenge. Pressure treated lumber saw
dust just doesn't seem like a necessary evil.

That oak top will want to expand and contract acrossed the
grain - even if it's all quarter sawn. If you screw MDF to it
the wood movement will either tear up the MDF or cause
the top of your bench to ripple - neither being desirable.
If you're going to use screws into the oak, ovalize the holes
in the MDF to allow for some movement. That'll require
pan head screws - the bottom of the head being flat instead
of tapered.

Having the inside jaw of the vise flush with the apron of the
bench is a good idea. Usnig the apron of the bench as a jaw
isn't - unless you're apron is going to be 4-6 inches tall.

As for the dimensions of the top - 30" is a bit wide. Most
things you'll make probably will have one dimension that's
24" or less. A five foot long bench on the other hand seems
a little short, especially if you want to make a book case.
On the other hand - an 8 foot long bench is a PITA to walk
around in a shop with little unused floor space.

Regarding the height of the bench - what kind of woodworking
are you doing now or are planning on doing? If the bench is
for mainly hand tool work you want to have the top lower
so you can use your body weight to help the tool. General
rule of thumb is to stand up straight, put your hands at
your side, bend your wrist so your palms are down and parallel
to the ground. That palm height is what to shoot for - plus
or minus and inch.

A workbench is a tool to help you work wood. It's a pounding,
chopping base, a giant, versatile clamp, and maybe an
assembly bench. Understanding what it can do and how those
capabilities meet your woodworking needs is worth the
time to research the options. Ideally, you only want to build
only ONE real woodworkers bench so think it through before
deciding on details - or vise hardware.

Here are two sites worth going through.

http://www.geocities.com/plybench/bench.html

http://www.workbenchdesign.net/

And if you want to go through how the one I built
came to be, in more detail than you probably want
to slog through

http://web.hypersurf.com/~charlie2/D.../CBbench0.html

charlie b