I was in the exact same situation as the OP (old HF 90a flux core box, wanted to weld thicker stuff, curious about TIG) and ended up with the YesWelder 205DS. I'm honestly THRILLED with it.
Couple of things to consider.. Any machine on 120 is not going to do thick metal well. Watt's law says 120v x 15a (typical breaker size on 120v circuit) = 1800w. ANY 120v welder is going to put less than 1800w of heat into the metal, regardless of price or brand. 240v x 30a = 7200w. Enough said! If you get a dual-voltage machine and plug it into 120v for convenience sake, just be aware you are cutting the legs out from under it and putting a hard ceiling on how much better it can possibly be than the $70 HF box (mine was $89 in 2009
). The jump from 120 to 240 is honestly far more important than brand or features if you are planning to weld thicker stuff (ever).
Another BIG deal for flux core is polarity. On the old HF machine, polarity is gonna be wrong half the time no matter what because IT PUTS OUT AC! It's unrectified. ANY mig machine that actually puts out DC will weld 'better' with flux core than the old HF box. I was shocked how much better I could weld with proper dc and polarity.
And lastly.. just because you think you're welding 1/4" with the HF box, doesn't mean you are welding it well. I got pretty used to doing preheat before welding thicker stuff with that HF box, and unless you get that part glowing red before you start.. the weld is superficial. I welded a 2" receiver (usually <1/4" thick but this one had one side thicker) to a 3pt frame twice with that HF box, and both times it broke off under hard use. I preheated it quite a bit and really took my time, especially the 2nd time around. You can stick 1/4" things together with it and have them stay together indefinitely if you don't really test it, but if something 'needs' to be 1/4 thick in the first place, it 'needs' to be welded better than what that box can do. I'm stubborn as hell but I had to admit the fallacy of attempting to weld that thing a 3rd time.
So yeah, get a 205DS! I have no regrets whatsoever.