Tight budget, and I'm a novice diy'er.
My wife wants my to build a wall to wall bookcase that is floor to
ceiling. There will actually be 3 sections because we want the end
sections to have workstation or desks. They will be fastened to the
wall. My wife and her family suggested I make everything out of MDF
which is cheep, easy to treat and easy to by decorative faces and
mouldings for.
However I'd like to seek opinions on how long 5/8" or 3/4" MDF will
last if two adults and two kids use this type of configuration for 20
or 25 years or hopefully 40 years?. What's the life span of MDF for
this type of application? We are in Ontario Canada so air
conditioning 3 months a year and forced air heating for 7 months a
year from a humidity standpoint. Is there plywood that is affordable
and nice to prime and paint and that we can nail some mdf moulds to?

First, forget MDF as a finish material. It will sag, swell with moisture,
won't hold nails/screws, etc.

Second, factor the intended life span of this project into your "budget".
Even if you spend an extra $100 now, that's only $5/yr over the next 20
years. Less if you think it will last longer (it should if built
properly).

My choice is 3/4" birch plywood. You can find it at any home center for
around $40-50 a sheet right now. Rip it lengthwise to 11-1/4" wide
strips. You can use these strips for the sides of the cabinets, or cut
them to length for shelves.

Build a simple box, as high as you want (up to the 8 foot length of the
plywood), and a maximum of 3 feet wide. Any wider and the shelves are
likely to sag. You can build as many "boxes" as you need to span the
room. I would divide the space evenly, so each box is the same width,
though you might want to leave an inch or two on each end you can scribe
a filler to fit the wall. Or, build it close to the full width and use
trim to cover the gaps at the sides.

I would use 1/4" plywood for the back. Normally I would inset the
plywood into rabbets cut in the sides/top/bottom, but since this will
fill the wall, you could just glue and nail it to the back of each box.

Then drill a series of 1/4" holes along the insides of each cabinet for
shelf pins. There are a variety of jigs to make this easier, or you
could use a piece of pegboard as your jig.

You could make face frames out of poplar (again, relatively inexpensive
and available at all home centers), to strengthen the boxes and cover the
end grain of the plywood edges. I recommend using pocket screws to build
the face frames, as they're strong and easy to make with an inexpensive
jig (I think Lowes sells them now). Glue/nail strips of the poplar to
the front edge of each shelf also, then cut to fit inside your case.

I would choose stain and poly over paint, as I think it looks better and
will hold up better to the wear and tear of dragging books in and out.
But finish it however you wish.

If your boxes are over 5-6 feet tall, I would probably add a fixed shelf
in the middle to help keep the case from bowing in or out. Then just
have adjustable shelves top and bottom.

Good luck!

Anthony