Offset is measured from the bolt up face of wheel/hub face to the centre of the wheel.
Backspace is measured from bolt up face of wheel/hub face to back edge of rim.
Plenty of good diagrams on the web, just search for "wheel offset backspace"
Remember with backspace measurements you need total edge to edge measurement of rim which in most cases is 1" wider. So 10" rims is measured at tyre bead and will actually be approx. 11" from edge to edge.
With caliper clearance you have to check that calipers will clear the rear of the spokes. A lot of wheel suppliers will provide a template or measurements for this. Also brake suppliers will also provide templates to help with this. They create brake caliper clearance by both rear spoke design & by building up the wheel hub. The more they build up the wheel hub, the less outside rim you end up with.
I know with wide deep dish Simmons wheels, like 12" wide wheels, Simmons make both outside & inside rims the same size and make up the dish by supplying a really deep centre hub. It's not unusal to see tops of wheel nuts 50mm or more below surface of the hub. So what you thought should be really deep dish wheels or not supplied as such.
And with Simmons and other 3-piece wheels you have to also take into account clearance on the nuts that hold rims together. In saying that you should be fine. Those brakes won't be big enough to cause problems with bolts.
Are Simmon's still using the same size centre on 17 & 18" rims?
I'm actually a bit more of a fan of US Billet Wheels these days. Both inner and outer rim are polished, centre caps are also billet and centres are generally made of higher quality alloy so hold a superior polished finish. And most are supplied with true size centre so you get that nice big centre/flat rim look. The irony of it is, most of the rims are sourced separately to the centres...so for example a Aus BonSpeed is not the same as US BonSpeed. Even look can be different as they skimp on wheel centre size.
Simmons are about the cheapest 3-piece wheels on the planet though and have a proven track record. I have had good success with Simmons in the past, just hope they have fixed up their custom paint offerings and we had to re-paint a set of Simmons wheel centres as paint finish was visually poor & also did not bind with the primer, as in you could scratch it off with your finger nail.