If you just want to buy a license, you can do that, but I would consider filling out the form as a potential service partner as a starting point. If they don't want you or there is something that makes you not want to do this yet, then fine, they can point you at a rep and just buy a copy.
As an end-user, you won't be able to buy licenses for resale, but, yes, your customer can buy them direct from PSC. I'm not sure whether you can resell licenses as a service partner. That would be one of the things to ask. This would be nice because you can make a few bucks on the licenses and tie your customer to you. Most of my customers historically have had very weak internal IT resources, so I give them all encompasing support contracts for the application and for Progress and then I pay their Progress maintenance for them (at a discount). That way, they call me with any questions without having to figure out if it is a Progress issue or my application and I can give them better support and a single point of contact ... while making some money on it.
One issue I can see with your plan is that getting a development license for yourself only gives you a personal DB to work against. For another $1000 you can get a 5 user workgroup license to work against. But, one of the big advantages of becoming a service partner, other than visibility, is that you become eligible for the PSDN subscription. $3K a year might seem like a lot, but consider that it means you have an enterprise database to work with, AppServer, Sonic, etc., etc. so that you can develop and test fully modern applications for your customers.