It depends what gauge are you looking for. Consumers make a simple mistake by referring just to gauge. The thickness of the sheet metal used to fabricate a sink is called its gauge. The gauge of sheet metal for most sinks ranges from 22 -15 gauge. The lower the gauge number, the thicker the sheet metal is. High quality residential sinks range from 20-18 gauge while commercial quality sinks are offered in 15-16 gauge. When manufactures refer to the gauge of their product, they are referring to the thickness of the sheet metal before the manufacturing process begins. Most stainless steel sinks are manufactured using a process called deep draw forming where a sheet of metal is drawn down into a form through a series of steps called reductions. Each reduction stretches the sheet metal deeper and deeper into the form, so what started out as an 18 gauge sheet of metal will not end up being 18 gauge throughout the entire finished product. The industry standard is to state the gauge of the metal that the manufacturing process started with. Since this is the standard practice, it makes it easy to compare one sink to another. If you are looking for 16 gauge finished product you need to look for 15 gauge sink, not 16. I hope this helps you with your selection.

I have a stainless drop in. I don't understand all the talk of crud catching in the lip, I don't find that to be the case. Regular cleaning around the sink area every day makes that a non issue for me.Having never had an undermount sink I can't say, but it would seem to me that more crud would catch in that area than in a top mount. I've never dented a stainless sink, I would guess you'd have to drop something pretty heavy in it to dent it. I guess it depends on how thick your steel is? But I do hate the water spotting on the stainless. Everything has its trade offs I guess. For the most part I find the drop in stainless pretty tough and easy to care for.

Since you're retrofitting, I'm not sure but if your fabricator has any experience with counters, she can probably weld in a sink you purchase that fits the hole (once you pull out the old one) or even fabricate you one from scratch. The last stainless counter I had installed wasn't really very different from what you're doing. My GC put down the substrate and the stainless guy came in and templated. It might have well have been an old laminate counter! Pin down the fabricator and ask if they can weld in a sink or custom fabricate one. It should be not much different from doing a job from scratch.

I am worried about reviews on sinks leaking. I assumed it was the welding that failed What sinks are you seeing reports of leaking? When there is a leak, it is extremely rare that it is the body of the sink that is leaking. If there is a leak under the sink, or around the sink, it is coming from something else, not the sink welding. I would give up welding as a decision point in my choice of sink. As @User pointed out, stamped sinks are thin, light weight. They are sure to sound tinny when you drop a fork in them. They are sure to dent if you drop a pan in them. They are sure to vibrate when the disposal is turned on. My advice is to concern yourself with the reputation of the manufacturer, with the gauge of the stainless steel (it should be 16 gauge), and with the configuration. Is there a reason you want a 45" sink? We have a 36" sink and it seems very large. Make sure when choosing a 45" sink that you do not have to forego something else you'd want, like more counter space, a wider fridge, wider cooktop, or similar.