Hi mate, I also like shooting landscapes and similar to yourself I am on a tight budget too.
The first thing to say is forget wide angle adapters, and macro filters (the screw on to your current lens type) as they cause too much distorsion and lower your lens quality. I wouldn't have these if they were free.
I looked into possibly using older lenses with an adapter as well, but personally came to the conclusion that the 1-2 stop reduction when using an adapter pretty much negated the benifits of buying an older lens, and seeing that most were made for film cameras focal lengths below 24mm are extemely rare. 24mm on a crop sensor body just wasn't wide enough for what I was looking for either in a wide angle lens.
Lightroom is great, but you can't create panoramas or HDR in Lightroom itself, you also need Photoshop to do that and that makes it a bit more expensive.
That being said there are a few things that I do use, that I'd reccomend:
1. Circular Polerising Filter - a good branded one is still reasonably cheap in photography terms and for landscapes it's brilliant. Don't leave home without one!
2. A remote shutter release, I'd buy a cabled one as you don't need to worry about batteries then
3. A set of ND/GND filters. I bought an
eBay special colkin P type filter set, they are not the best quality but are still useable, and not being the screw on type they will fit different lens sizes
4. A gorrilapod or other small tripod, you can often get these in places where larger tripods are not allowed, and with a bit of creativity they can be very effective. I fix my gorrillapod to the outside of my small rucksack and take it on as my hand luggage if I'm flying abroad.
5. Extension Tubes. Probably the cheapest way to get into macro photography though a bit of a learning curve to learn to use them effectively. If you want to do really close up or moving insects you'll find out quickly you will be wanting an off camera flash setup.