Typically, you'll need a holder, and adapter ring that threads onto your lens barrel (in your case a 77mm ring), and the filter holder itself. I wouldn't recommend a single-slot holder such as the Cokin P wide anglefor the simple reason that you will not have the ability to use more than one filter at a time. There will be occasions when you will need to stack filters, and this type of holder will prevent that ability. Also, with the Cokin P WA holder, you'll need to carry along the regular holder as well.
I'd agree with your use of soft edged filters if you intend to have objects above the horizon. There are many varieties to choose from...Cokin, Hitech, Lee, and Singh-Ray. These are also listed in the order of price, from low to high. The best value, IMO, would be the Hitech line of 4x5 filters...very good filters at a moderate price. The Cokins tend to leave a colorcast on the image...this can be removed in PP, but who wants something else to deal with if you can avoid it in the first place? Another filter to consider is a "reverse GND". AFAIK, only Singh-Ray has these (but I could be mistaken). If you plan on having the actual sun in your sunrise/sunset pictures, this would be an excellent filter to have along. Unlike a regular GND, the reverse filters are darkest at the center and fade towards the top. These are a bit pricey.
As far as holders go, personally I'm partial to the Lee holder. Excellent design, and no little pins to fuss with such as with the Cokin Z-Pro. Its also customizable in that you can adjust the number of your filter slots as necessary (can't do that with a fixed holder). And you'll only need the one holder...can be used on both wide angle and longer focal lengths with the proper adapter ring.
If you decide on the Lee, all that you'd need would be (1) the holder itself (known as the Lee foundation kit) and (2) the 77mm wide angle adapter ring. You should be able to use the WA ring with both of your listed lenses. So...one holder, and one adapter ring. I'd start with two filters, a 2-stop and 3-stop.
A good source is the following link:
http://www.2filters.com