I think the best way to go is build a smaller RepRap style printer for around $300 that uses Nema 17 steppers, then use that to print the needed parts to build the larger RepRap that would meet your needs. You would be able to reuse the steppers and controller board from the smaller version that you use to strap the larger version, and many parts are lower cost to print than buy for the larger one.

If you use a non heated wood print bed, and 8mm rails, then getting the longer rails and bigger print bed is not a problem.

Your total should be about $300 max for the first smaller printer, plus about $20 for rails, wire, and wood to expand it, plus about 0.2 KG of filament for the printed parts at $21 per KG that would be $4.20 in filament, so the total would be somewhere under $325.

Oh I left out the BB's that would be used in the printed linear bearings and ball bearings (maybe 5 cents worth of BB's).

Going this way would also give you some direct experience with 3D printing before you get to the big one.