I think you really need to assess what products you will be offering. If you don't know the questions you are asking about, what value will you be giving potential customers? Your state will have laws/codes/regulations on surveying and if you have yet to even look into them, I think you are setting yourself up for purchasing equipment you don't need to offer products you cannot legally sell.
You should look into selling "maps" that show an area for inspection, current conditions and for that you do not really need the high end GNSS equipment.
If you are having problems now, you haven't even reached the other major problems in learning datums, coordinate systems, ellipsoids, geoids, orthometric heights, local grids and then applying this to your drone, GNSS Base/Rover and photogrammetry software of choice.
All can be learned, but there is a timeline and curve and a forum thread while useful on specific questions will not be enough to teach all of it.
To actually answer your question, There are several "cheap" options to collect control points. A popular choice are Emlid RS2 and RS2+. Whether you need 2 units or one depends on if your state offers free NTRIP corrections, or if you want to use PPK workflows.
If your state has free corrections, then you will need one unit to collect control points, and if you choose to purchase an RTK drone, the drone could receive these same free corrections.
If your state does not have a free service you could use PPK and download RINEX logs from NGS CORS stations to make your collected control points and logs from an RTK drone (if purchased) accurate after/post mission or point collection.
If your state does not have this free service you could purchase a subscription (not cheap and recurring cost), you could buy 2 RS2's and use one as a base and the other as a rover to collect control points. This base can also send corrections to an RTK drone.
Above is a quick summary, and there are some more combinations you could use in workflows.
Also there are other multi-constellation, multi-band receivers out there besides Emlid that are lower in cost, but I use Emlids and highly recommend them as they have great free software that increases in capabilities with new updates constantly. They also have IMO the easiest to use cloud caster service for sending corrections.