I would recommend you read my article on the SurfPrep sanding website on 2 head wide belt sanders. If you spring for an SCM, which is a great machine by the way, I would get one that has air exclusion on each of the three sanding contact points. This means you can retract either of the drums or the platen instantly.
The first drum will be steel for cutting things flat and planing. The drum in the combination head will be 40 shore for a longer more shallow scratch. The platen in these machines is usually 50mm wide (2").
80, 150 not a good sequence but it's not far off.
Running a part through the same grit sequence is actually not a very good idea. If you sand a rough surface down to 150 grit the scratch is only a few thousandths of an inch deep. When you do a second pass on the same surface removing the same amount of material, the second pass is removing solid wood. Maybe up to twice the actual mass of wood is removed from the surface.
It is very important to not only knock the surface down flat, but also to leave a proper scratch pattern for the finishing sequence to remove.
I would much rather see you run a 100 - 150 finishing sequence. Use at least a 60 grit by itself on just the first head to knock it down flat on a few passes and then use the 100 - 150 to remove .015" to get rid of the 60 grit scratch.
This is a real quality way to get your job done.
If you leave .015" per side to finish, you have .095" of your .125" left to remove. Even with a smaller SCM you should be able to remove that with 3 passes at 60 grit.
The reason this is important is because the final grit sequence will be removing a scratch pattern, 50% of that is air. The final sequence will be under very little pressure so the belts will run a very long time. Your color for stain will be excellent and reliable.
This is a lot to take in so feel free to send me an email and we can discuss this in person.