If it was me... I'd be looking for a used Hardi and make sure it's got the Hardi diaphram pump.
Roller pumps will make volume but they're limited to spraying certain types of products. That might not be an issue to you if you're only spraying liquids... but you're also probably going to find the roller pump stuck every year when you go to use it, seals leaking, etc. They're just a nuisance to deal with. Their only virtue is that they're cheap.
A hardi pump will last a long time and take a fair bit of abuse... I wouldn't worry about a used one unless it's got water running out when you fill it.