I have worked for a buy-here-pay-here dealership, a sub-prime auto lending company and a buy-here-pay-here software company. I should be able to answer any questions you have. I find that choosing good software is key with buy-here-pay-here. If you can get good software and a good web presence, you should do well.
The software does vary a lot though. You get the more basic software like the Frazer product that will let you track loans, but not easily or that well. You get good software like AutoPal Software that does a ton of things like let you set up automatic withdrawal, transfer transactions to quickbooks and provide a customizable website. You can also go really big with companies like Gold Point Systems that will create a custom solution.
Typically though, as you company gets bigger, you will basically want the software to run the company so you can focus on marketing. I would say that your location (how it looks, what materials you have about loans, etc.), then your online advertising (including inventory and information on your loans) and finally local advertising, in that order, are best for marketing.
