For a boil vessel, look to get a keggle... That will work for both 5 and 10 gallon batch sizes. Especially since you're on a budget. I plan to get one made into a mash tun at some point, so that I can directly fire it to increase the mash temp (to get the grain bed up to sparge temp without adding more water).
I have an 8 gallon aluminum stock pot converted into a kettle, plus a 10 gallon Blichmann kettle now. I have a propane burner, 10 gallon and 70qt cooler mash tuns, plus grain mill, plate chiller, and other assorted gear. I've lost track of how much I've spent, but I consider it all money well spent. I do wish that I had listened to people telling me to get a plate chiller early on. I went through the cheaper/easy to make IC phase, before getting fed up and going to the plate chiller. I can't see ever going back now.
There will always be some piece of gear that you'll see and want to get. Don't think that you'll be set for the long term with your initial gear purchases. Once you do start brewing 10 gallon batches, you'll probably need to adjust your hardware to reflect. Unless you have a monster IC, you'll want to get a good wort chiller. Plate chillers work by chilling the wort as it goes into fermenter. You can get one that will work with your ground water temps, and get the wort to your desired temp easily. I picked up the Chillhog 4000 and RebelSmart from Rebel Brewer. This combo allows me to see the temp of the wort leaving the chiller and adjust the flow to make it hit my desired temperature. It doesn't get much easier than that. Well, it will once I add a pump to the mix. :rockin: