I would look at the New Members forum where MistressNomad has made a series of great videos for beginner vapers and vaping on a budget. They are GREAT! They will tell you everything you need to know about beginning vaping. With a $100 budget, you can do quite well and probably have money left over. I ended up spending about $90 when I ordered my started kit from AquaVapor. One of the main reasons I chose them was that they were local to me and I liked the idea of being able to see someone in person if I had problems or other issues. But, I recently ordered supplies from China (which I was VERY heitant to do, but I couldn't resist the price). Everything arrived very quickly and I would definitely order from them again (Bestecig.com).
I would recommend a 510. I have both a 510 (that I ordered from AquaVapor) and a KR808D1 that I ordered from Vapor4Life. Of those two I would recommend the 510. The 510 also seems to have the most options for third party supplies to save you money over time.
As far as refilling - DEFINITELY YES!!!! It is SO easy, especially with cartridges. But, I prefer cartomizers (the cartridge and atty in one unit). They are also very easy to refill. And if you ever have any questions, check the forum, or just go to YouTube and there are tons of videos - if you are a visual person and need to see it done - I know I was). If you opt for cartomizers, make sure to order extras. If you decide to use cartridge + atomizer, make sure to order an extra atty, as they will wear out. I find that cartomizers give more flavor and better TH.
I wish I had a great juice to recommend, as I am still trying to find the perfect one that will get me off analogs 100% (but vaping has helped me decrease my analog use - I smoked about 1 to 1 1/2 ppd of ultra lights). Thus far my favorite is Velvet Vanilla from Velvet Vapors. I ordered 50 PG/50 VG blend. She has GREAT sample pack options where you can choose 5 flavors and each one can be a different blend of PG to VG. You also get to select all your flavors from her endless library. Many companies that offer sample packs have the flavors already selected for you. My second favorite is the VG WOW vanilla from V4L. Not much of a TH, but great flavor and vapor. If you are looking for a nice tobacco flavor, I would recommend FreedomSmoke USA's Wyatt Earp. They included a sample of it with my order and I had heard it was nice and I was pleasantly surprised.