ernie always says the thermal arc 185 is the best value. i wish i bought it instead of the lincoln i got (squarewave tig 175), though, i'm satisfied with the lincoln but it weighs over 200 pounds, would be nice to have the capability to EASILY transport it. i'm not an expert here, just putting in my $.02. from what i gather you can make serviceable welds w/o the pulse feature (and save yourself some money not buying the pulser) but i think the pulser allows you to more easily make prettier welds (stack of dimes bead). oh, was going to ask if you are subscribe to sci.engr.joining.welding but i see you are. ernie is generally regarded as the resident uber-guru, if you ever expect to get any welding advice on these lists in the future (which you inevitably will). :-) you'd better beg forgiveness, he's a font of invaluable free information and advice, (and is well regarded amongst the other denizens on these lists)
Application is a tube chassis vehicle: No specification given for the tubing. Is this round tubing or square stock? What diameter or dimesions is the tubing? What is the wall thickness? A little more information could help clarify what weld process would be best.
Some have suggested MIG, but MIG runs very quickly and can be difficult to make good welds with on round tubing. Particularly if you have little experience welding pipe. You may find that consistant weld penetration is difficult to achieve as you constantly have to modify the gun angle when you work your way around the tubing. Or, you may over penetrate from not being able to adjust quickly enough and blow holes. Further, you will probably need to stop the weld at least once, rearrange your position, and restart the weld on the other unwelded half of the joint. Without properly grinding the preexisting weld ends, you may have a serious weak spot in the weld where it laps the preexisting welded area.
If you do decide to use a MIG welder, I would suggest using flux cored wire. The stuff runs very smoothly in all positions, has good penetration, and even if you have little experience with pipe/tube welding, a little practice should have you making acceptable welds.
Since it sounds like you need high quality welds with a lesser chance of cracking or breaking due to vibration in the finished vehicle, I would go with TIG. Try to find weld inserts suiable to your base metal if you use TIG. This makes for a great method to produce a high quality root pass. TIG welds will, however, take a much longer time to apply.
Joint preparation is KEY in this type of welding. Read up about how to do it for the stock you are using and the process you are using. Dave