Well, that will depend on what type of building you want to create. Things like restaurants take more time/trouble than a shelter for an animal. What you absolutely will need are
1) WinZip or WinRar (programs with free downloads)
2) dxtbmp (free program, I believe)
3) a good painting program (PSP or Adobe) both have free trials; possibly even GIMP (a foreign-based free painting program)
4) a hex editor (there are many available, such as WinHex, HexFlex, and Cygnus) none of which are free except for trials