If you are just getting back into photography, I would suggest going cheap (even the kit 18mm -xxx lenses) and wait to see what you like shooting first before dropping a bunch on glass. Programs like Lightroom can show your most used focal length for a group of photos - that is what many use to see where they should put their money towards.
That said, for a wide angle, I use the Tokina 12-24 ver 1 with the screw mount. It is tack sharp as with the rest of their newest lenses and it is built like a tank. They are the only 3rd party that maker that rivals Nikon & Canon. You can pick one up easily for $450 and it will be just as sharp as Nikon's. They also hold their value quite well for resale. Gabbb is the first person who I have ever heard they thought the 12-24 was a bad lens. There are quite a few who do, or have had that lens.
If you are looking at upgrading in 3-5 years, I wouldn't bother with FX yet on the wide range. Twice the size, twice the cost, twice the weight, no real advantage over DX and who know what will come in another 3-5 years.
Honestly I have never shot landscape photos below F8 and you can handhold a wide angle lens at f/4 down to 1/20th. I think 2.8's are a waste unless you are going to do some wide angle, very close up shots and try to throw the background out or maybe rock concerts.
Last thought, I rarely use the 12-24 for landscapes. Cityscapes, indoors, odd angle stuff but rarely landscapes. I like the "normal" 35-75mm view (fx equiv) and the compression from 150-400 (fx equiv). Just food for thought and to round back to my first comment. I would see what you like shooting before dropping down a bunch of money. You may find you only like long telephotos, or maybe you want the widest possible you can git like the Sigma 8m-16mm.
Let your experience show you, what you need.