you are on a budget so throw out the 4 wheel disc idea to start.
front discs will do you just fine.
i did mine with factory parts i pieced together
there are also alot of kits out there.
find out ahead of time what the consumables will cost to replace
and thier availability like rotors and such
it would suck to buy a kit you can't get parts for
or the parts will cost you a arm and leg to replace when they wear out.
thats why i stuck to the factory stuff myself.
it works just fine.
also before you start you need to know what size rim you are running
this is very important as
alot of kits or factory parts will only fit certian sized rims.
so you need to answer those questions first
before you start looking for/ buying parts.
here is a very good article to read before you start
all about a factory front disc brake conversion
and the parts you will need to pull it off.
http://www.moparaction.com/tech/archive/disc-main.html