Replacing Interior Doors

ask your super

Like nothing, and solid core is a good choice. Knobs can get pricier than they should. Make them replace the overpainted hinges for some extra sparkle. They won't want to.

Trustile offers all manner of "solid" doors from slab to panels and varying thickness. Nickel hinges are a nice touch with matching knobs. A solid feeling knob is a nice indulgence (a must-have of mine): check out Samuel Heath, for instance.
http://www.trustile.com/
http://www.samuel-heath.com/products/products/door-and-windown-fittings/door-furniture/internal-door-fittings/Door-knobs/747/

$60 door: http://tinyurl.com/c2sjt34

$30 knobset: http://tinyurl.com/btvdqd6

... enjoy feeling like you're not jonesing in 1950s housewife suburbia.

MDF doors would cost you around $350 and more depending on height, panels and thickness. Each door would require at least 3 hinges. Decent quality hinges are about $8-10 each. You most likely would need to install new knobs or levers. More or less decent ones start at $40-50, something nicer $80-90 and up.
MDF doors are very heavy, so you need at least 2 men crew to install it. When you spend this much for the doors you may also consider a pro paint job. So it quickly adds up.