You might be surprised what you can find on ebay if you are patient. My father lives 3 hours away from me in the LA area so that opens up a lot of possibilities for me. A few years ago I paid $200 for my drill press on ebay, it is an early 1960's delta unidril which is a little known model of radial drill press. The table is 24" square and weighs 150lbs before adding the 90lb legs. all told it is just north of 400bs and with the drill head able to swing compleatly out of the way it makes for a great small heavy duty worktable.
I had been on the look out for a wood planer to replace my aging 12" lunch box paner and after about 5 or 6 years I found a compleatly restored early 1940's parks 12" planer 5 miles from my house on the local craigs list for $225. This is a 350lb heavy duty precision piece of equipmment and was exactly what I was looking for with the exceptuon that it had already been restored by a retired machinest. To top it off I was able to sell my tired old planer for $50.
I'm definatly not a fan of casters for any tool that is powered and has a sharp blade that you will be near. for things like welders or torches casters are great. And this is from someone with a very small shop. carefull planing and using overlaping infeed/outfeed areas and having as many fixed table heights at the same level makes things work smoother in a small shop.
For a work table 2x4 and 2x6 works fine. Glue and screw the frame but atach the top only with screws so you can replace it when it gets tashed. I prefer MDF or even particle board over plywood Ply wood splinters and with 2x4 suports every 24" mdf is plenty strong. a common mistake is to make a work table too tall. You should be able to stand comfortably at the bench and place you palms flat on the table with your arms straignt at your side. Add or subtract an inch depending on the work you intend to do at the bench ie. rebuilding carbs for a living you might want a taller bench but for splitting engine cases you might want a lower bench. The nice thing about working on an engine on a wood bench is that you will not scrratch or mar any soft aluminium parts.
For welding you might also consider an old tig. The Airco/Miller and Hobart units are just about bomb proof with contact points that are so robust that they should last a life time of filling and re setting. These units are huge compared to a modern tig and they lack square wave technology but they have a very robust diuty cycle and are supper reliable. they are also pretty decent stick welders. If you are truely interested in a stick welder don't hesitate to snag an ancient Lincoln 'torpedo' if you find one. they are usually worth more than the asking price just for their scrap copper value and they are one of the smothest stick welders you will ever find bar non.
Don't be rushed, tell anyone you know what you are looking for and have fun. oh yeah dont forget tons of light and good tunes.