Thick steel (3/4 inch and up) can be Arc (stick) welded or Wire welded (most easily with a large wire feed welder 200 amps and up) wire welding includes MIG and Flux core wire. Steel this thick will typically be preheated before welding usually using a propane or acetylene torch.
Sheet metal is hard to arc weld. If you intend to weld sheet metal an arc welder is a poor choice. In any case if you go with an arc welder as has already been pointed out, a DC welder will be a better choice.
With sheet metal: MIG, flux core wire and TIG are more appropriate techniques than Arc welding. MIG will handle sheet metal easily (for example the Millermatic 251 will handle steel as thin as 22 gage steel. The larger MIG welders have both more power and the capability to use larger spools of welding wire. The Small 110 Volt 100 Amp wire feed welders can only use the small 2 pound (4 inch) spools of welding wire. The larger 210 and 250 amp machines can also use 10 pound (8 inch) and 30 pound (12 inch) spools. There is a far larger variety of welding and hardfacing wire available in the large spools than in the little .
With TIG and a welder capable of low current operation it is possible to weld extremely this metal. Figure that with TIG you need one amp per thousandth of an inch so 250 amps are needed for 1/4 inch steel. With sufficient low current control a TIG welder can weld (A welding instructor who posts on the Internet has a couple of tests of low current TIG welder performance and control. First he welds two soda cans base to base.Second he draws his name on the side of a soda can with the TIG torch. In both cases burning through the aluminium can fails the test. )
The preparation of the steel will depend on the technique you use. MIG and TIG require clean metal, rust and paint should be ground or sanded off before welding. Arc welding tolerates much more contamination. Flux core wire welding is in between. The weld process must have a way of clearing contamination from the molten metal of the weld. The Arc rod is coated with a layer of flux which combines with the contamination and oxides and floats the contamination to the surface with the flux. The flux is chipped off after the weld cools slightly. MIG and TIG rely on antioxidants alloyed in the filler metal as well as the inert gas shield to reduce the quantity of new oxides forming.
So for building and repairing heavy ground contact equipment you have ARC and Wire Feed (MIG + Flux Core) to chose from. ARC will be give the easiest preparation. If you will weld on existing rusty implements, you can wire brush off the lose rust and weld. With wire feed you will need to clean the metal far more. MIG will require shielding (a tarp tent) if you will work outdoors to keep your shielding gas from being blown away by the wind. For both techniques you will need to bevel the joint by grinding or gouging.
Hardfacing will also be an interesting process for building implements. Hardfacing is laying down a layer of material with desirable characteristics on the surface of material. This is useful to lay down a layer of abrasion resistant material which will be in contact with the ground or to replace the material which has worn away already. The hardfacing material is primarily available as rods for arc welding and rolls of wire (with and without a flux core) and to a much lesser extent as TIG rods.
My interests are similar to yours. Last spring after years of procrastination, I finally bought a welder. I ended up purchasing a Miller Maxstar 200 Welder. With this welder I can TIG and ARC weld steel at up to 200 AMPs. This gives me Arc welding for thick steel (though my wish list only extended to 3/4 inch plate) and hardfacing and TIG welding for thinner steel (almost up to 1/4 inch). I also purchased a Hypertherm 600 plasma cutter. With the plasma cutter I can cut and shape the metal I will weld and gouge thick metal.
(I am ignoring the fact that a skilled welder can arc weld sheet metal and even weld thick plate with a 110 Volt MIG welder, this takes real skill and experience. )
There are a lot of recommendations and commentary on the selection of a welder on the net.
You will find several discussions on welding in the Build-It Yourself forum here.
The newsgroups sci.engr.joining.welding and rec.crafts.metalworking are great sources of information. (If you don't have a newsreader set up you can access them through google
http://groups.google.com/groups?hl=en&lr=lang_en&ie=UTF-8&newwindow=1&group=rec.crafts.metalworking
Hobart Welders has a message board http://www.stagesmith.com/Metal-links.html