This article here should help.

Take your time as there is not one size that firs all, plus it is a long learning curve, so make sure you fully understand Midi capabilities first.

As to interfaces then just like the computers it depends on what you have to spend, and what features that you require, (The article should help) which only you can decide.

Finally make sure you really want to go down this route, as there are some great hardware workstations out there, also don't think it's as easy as using an arranger which does pretty much everything for you with just one button press, you have to get down and dirty with Workstations and DAWs.

Bill