It depends on the locomotive. Weaver brass typically used the Seuthe smoke unit. Of course, knowing exactly what you currently have would be useful. You're asking about things like how to remove the boiler shell, but we don't know if this is Weaver Brass or diecast.
If you have the Weaver model with the TAS (Train America Studios) electronics, adding cruise is a significant undertaking as you'll have to replace the existing electronics. If you have one with the Lionel electronics, then you can fairly simply add the ERR Cruise Commander M to give yourself cruise.
Why not grab the shots and description from the eBay listing and post them so we can see what you got? Failing that, how about just posting the eBay auction number (note direct links to eBay are not permitted here).
