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? Melamine is prolly out of the question cause of
looks and it's really tough to paint ( it's a home office library, not a
kitchen)

This room will be used a lot we think.