Alex Goldman of JupiterMedia, ISPCON and ISP-Planet blogs about not buying an ISP or WISP today. Instead, you will likely get more bang from your buck on buying a skill. In other words, add services to increase ARPU. Add services to increase stickiness. You want profitable customers. If you take away churn, customer acquisition costs, and marketing expenses, but increase the amount your customers pay you, you could be better off than just buying more customers.
I haven't seen too many WISP purchases make the buyer happy and profitable. In many cases, a WISP is like a boat - your favorite days will be the day you buy it (open it) and the day you sell it. (There are exceptions).
Buying local ISP's with the same model as you (for example, buying another DSL reseller or dial-up) means that economies of scale can work for you. In the case of a DSL resellers, how can you even afford to buy them?