I use my 6x24 ruler the most, but wish I had a 6 1/2 x 24. The Omnigrid rulers have the angle markings and are easy to use for a variety of cuts. I also have a 6x12 that I use quite a lot. I used only these 2 rulers until I was able to collect some of the "nice to have" squares and triangles. If I had to start over on limited $ (it always is) I would buy a 6 1/2 x 12 and a 6 1/2 x 24.
Regarding mats, I have Olfa, Drits, and Kai. They are all quite forgiving but I like the Olfa best. NO mat will last forever, but with reasonable care they will last many years. One of my Olfa mats has been cut all the way through (I was cutting photo mats with a mat cutter on it) but I can still cut fabric with a rotary cutter on it. Doesn't do as good a job as it used to, but it does work.
Have fun in the US, and be prepared for HOT weather. Everybody is suffering, but temps in Phoenix won't be down until October. If you will be in the mountains it won't be quite as bad, but we are seeing high 90s here in Prescott.