I have worked with all sorts of VeriFone terminals from Tranz/Zon to Omni 3200, to Verix/VerixV and eVo (3740/3750, 3730/510, 570, and 520). Unfortunately, I am not familiar with the E315, but I suspect my answer will apply to it as much as it would to the other platforms (you may want to follow up with a VeriFone rep to make sure-I'm sure he/she would be very happy to discuss it with you if there is a chance you may be buying some more of their hardware).

1. All VeriFone terminals I have ever worked with are just empty hardware-you have to provide all the software in order to make it work. There are various companies (and I think VeriFone is one of them) that have already developed software for the terminals that will handle credit card processing and/or check processing. I have never done this, but I've seen them in use in in many stores. Alternately, you can write your own program which will include all user prompts, all communication to a remote server that will do the actual processing, all the code for printing receipts, etc. (this is what I do). Obviously, this is a lot more work, but it gives you all the flexibility you could want.
2. That really depends on what you are trying to do, but probably. We actually do something like this-we read card data on the terminal and then send that card data to a server which in turn connects with one of a variety of other platforms we have integrated with and it all works swimmingly.
3. Again, I'm not familiar with the E315 or its platform, but the SDKs for the other models have all come with lots of documentation including, in some cases, sample code here and there. Again, contact a VeriFone rep and see what you need to do to get access to their developer's web site, "DevNet".