I jumped from the little 5 gallon kit kettle to a Bayou Classic 44 Qt stainless pot with a steamer basket that did brew in a bag with. 5 gallon paint strainer bags fit perfectly in the steamer basket and I could do roughly up to 1.060 OG 5 gallon. I have since ditched the steamer basket and just got a custom made BIAB bag which works even better (have done up to 1.070 without issue). I also got a 62 qt bayou classic to do 10 gallon BIAB batches with, and just use the BIAB bag made for Keggles.
I have been using these for about 5 years or so without any issues, and really like the quality, and you can find them for pretty good deals if you keep your eye out. I have also used a keggle with a false bottom that I found locally, and use it with the others in a 3 vessel set up on occasion, but I would avoid the keggles unless you get a great deal on one already built the way you want, they are just much heavier and harder to clean than they need to be, but that's just me.
I purchased weldless valve kits, thermowells, and sightglasses to install myself, and customized them to fit my needs, but I really enjoy doing that kinda thing. I have seen a couple that come with at least a ball valve on the bottom.
Anyhow, thats my .02. I hope you find something that works best for you and will last you for years to come!
Cheers,
Nathan