If you are only printing one part in a single colour you can probably get away with less spools. When I first started I tried to limit the number of spools I bought and use refills. Looking back it was really more of a hassle and I had wished I had just bought more filament with spools at the start. The extra couple buck per spools would have made my workflow easier and I didn’t have to spend time/filament printing my own subpar spools.
I will warn you that once you have a printer, your options really open up and you will end up printing other items and using different materials. It really is a game changer. I would also suggest you look at getting the AMS for the sole reason you can load it with filament and it will auto refill ie move to the next spool when a spool runs out. This is really really helpful when you are printing a BUNCH of one part.
PETG - yes I would dry it in a quality filament drier or by using your printer to dry them. I would not suggest trying to dry them in your oven as it is easy to melt spools that way.