What's a good intro welder for subframe connectors?

As long as your welder is heavy enough to have penetration (and your electrical service as well) You should be good. Most 130 amp wire feed welders (115v.) will weld up to 3/16". Floor pans and the subframe connectors are thick sheet metal so you should be good to go. I sure like my MAC 130 (made by Miller). Hobart and Century are good quality as well. I sold MAC/Astro welders for years. (made in Italy) They were really quite good for the price. 110 and 130 amp units. THey got to where parts and consumables were getting hard to come by. Something else to think about. You WILL need tips, gas shields and possibly liners. Brand name welders are easy to get parts for. Off breed ones not so much. Make sure you have good electrical service. You wil need a strong 15 amp 115v. service and no extension cords!