The cheapest material to use will be mdf, i would make the frame out of 100x50 wall framing timber & clad it with MDF & paint it. anything painted will scratch & possibly deteriorate quickly if not looked after though like anything else.
If you can, use plywood instead of MDF, it is alot stronger & won't soak up anywhere near as much moisture as MDF but is also 3 times the price. MDF will absorb moisture, it is not pourous so will hold it & go "fluffy" & will continue to deteriorate over time so depending on how careful you are (we all get water on the stand from time to time whether it be drips or larger spills) you may need to replace panels in a couple of years which may also require emptying the tank & taking it off so maybe best to spend a bit more now & save hassle in the future. When i started i bought jebo heaters as they were cheap, after two years all the thermostats stopped working so i have replaced them all. I did get 1 second hand jager heater when i started & i have never had a problem with it & it still goes well, they cost alot more but really it is worth it getting good equipment first time if you can.
here is DIY stand instructions that will help
http://cichlid-forum.com/articles/diy_stand.php