You don't need to buy another pump. The chiller can be plumbed right onto your return line, as the last piece of equipment before the tank.
Drop in chillers tend to have reliability issues with that flexible hose... I've never bought one myself.
For a 75g, a 1/5 will work well, a 1/4 will give you room to go to 125g in the future. Always consider the future upgrade because chillers have a low incremental cost.
The only chillers I know of that can accept a heater are the Prime chillers... I'm not familiar on their quality, but the heaters that fit them are Hydor Theos which are total crap. Personally, I've always run my Jagers in the sump.
A dual stage controller is a nice to have but not necessary. Rancos are the best controllers for heaters, though a full blown controller like a Neptune or RK are also very good choices as they do other things too.
In a larger sense... the controller options have pros and cons.
With an external controller, you get somewhat more accurate temperature control, and the controller can also run other things like heaters. However, because the controller involved a probe remote to the chiller, you risk catastrophic freeze up. With an external controller, the probe is not mounted inside the chiller but somewhere else as you mentioned like the sump or DT. If the pump feeding the chiller fails, and the controller calls for cooling, the chiller will switch on with no flow. It will begin cooling the water in the heat exchanger, but with no flow, it will just keep cooling that water until it eventually freezes. This WILL destroy the heat exchangers (and in all practicality will destroy the chiller). That's the risk. You can counter that by installing a flow switch on the chiller's plumbing that shuts it down in a low flow condition, but this adds about $100 to the cost.
I've always bought chillers with internal controllers. The JBJ Articas are very nice with internal controllers. If you want to go wtih an external controller, Aqua Logic makes the best chillers, period.
