With this 19" 3U case the amp will look like a 100w class-A power amp.
A better idea would be to separate the PSU from the amp.
Also, 100VA toroids aren't giant, I wouldn't be concerned about mounting them with 4 or more bolts.
Depends on how thick the plate is. Dissipante panels are only 1.5mm I think, so this makes sense. Wouldn't be concerned though mounting the 100VA to 3-4mm thick panels. But don't use countersink bolts, only buttheads so you do not weaken the hole. Will be on the underside anyway.