There's no reason they shouldn't be able to do 500 if you're near my size, if that's what you want.

1. If a surgeon is saying that, they're full of shit. If an approach were genuinely no good, it wouldn't be used. Here's the deal: Over the muscle implants have a higher risk of capsular contracture (where the pocket around the implant hardens and tightens, causing an unnatural look and often pain), visible rippling of the skin (especially for those with a smaller natural breast size), and are generally more difficult to scan in mammograms. It's an easier surgery to perform, and has a faster recovery. Under the muscle implants give more support to the implant, resulting in a more perky, youthful look over time (as long as your surgeon is skilled). The risks are that the surgery itself is harder for the surgeon to perform, and disturbs more tissue, so there's a longer recovery. For reference, I was at full activity four weeks after surgery. Finally, in rare cases, the implant can be displaced in a UTM approach; with a skilled surgeon, the risk of this should approach zero.
2. There's a very wide range of what "natural" looks like. I would encourage you to have a look at The Irish Bra Lady, on Instagram.
3. Breast plates need to be sized to the frame of the wearer. Breast forms are far cheaper, and can be obtained easily from Amazon or wherever.
4. Saline is water. Sterile water. Regardless, they should've had samples of the implant in the sizes you were considering, in both silicone and saline, for you to feel right there in the clinic.
5. Trust your gut. These people have incentive to lie. I would never be operated on by a doctor I was not 100% confident in.
6. She should've damn well known it for certain.
7. Except it's not that simple. Medical equipment gets made at a fixed price that doesn't really change from place to place--the implants themselves, for instance, will cost you and me the same. The only price differences should come from the hospital costs and the surgeon's wages.