Welcome back!
If attending tournaments, you will always need the most up-to-date rules and profiles. You have the basic rules for the game and all the Hobbit-era profiles, but will indeed require the 5 sourcebooks (Free Peoples, Fallen Realms etc.) for the LotR era models. As the game will receive some updates later this year, I would recommend just picking up 1 or 2 sourcebooks for armies you would most want to start playing with - then see what happens in the near future and expand accordingly.
If you are in the UK, I would recommend joining the Great British Hobbit League Facebook group - it has all the information on the many upcoming events, throughout the country. Here's the latest schedule for this year:
https://drive.google.com/file/d/0B9KPDU ... MxY3c/viewIf you have a basic understanding of the rules, you can pretty much turn up at any event and have a fun day - quite possibly win a game too, or otherwise at least learn a lot and meet a bunch of people. As tournament games have time restrictions, you will want to have some vague knowledge of what to do, but by the sounds of it you'll do fine. Most people are more than happy to show you the ropes and are only happy to see more people join the fray. As you can see in the calendar above, the UK scene is divided in so-called 100 and 80 point tournaments; a number referring to the points that can be earned in the national league (nothing to do with points size of the games). If practical, visting an 80pt tournament will be an especially great way to start, as them being 'worth' less means they tend to be a bit less competitive.
Finally, the eternal answer to any 'would having this or that model count as that profile be ok' is: ask the tournament organiser. Generally speaking, proxies (using unaltered models to stand in for others) is not allowed, to prevent confusion and so that people that did invest much money in the official models or time in converting them won't feel bad for having done so. How much conversion work would be required depends; the effort shown tends to be an important factor. Some tournaments are very strict, other organisers will allow anything that looks cool (e.g. a massive Warhammer spider using the dragon profile to represent Ungoliant, a big swan as a great eagle in a Dol Amroth list and even weirder stuff). Just small alterations also tend to be allowed if it concerns figures that have never been made or are currently out of production.