Hi Andy,
I'm in much the same boat as you, just an extra month down the line! So here's what I found out that you'll want to know:-
On eventsLOTR went through a bit of a dead stage for a good chunk of the last decade, but over the last two years has been making quite the revival. A lot of people are in the same boat as you and me, and thinking back on it with fond memories, are returning to their roots I think. The result has been a flourishing independent event/tournament circuit based heavily around an organisation called the Great British Hobbit League (or GBHL) which runs things to do up and down the country. One of their most recent events in Stockport had over a hundred players! You can find them on Facebook here:-
https://www.facebook.com/groups/472128942815892/?fref=tsI'm planning on making my way over to the East Grinstead event in a month and a half or so, and am quite looking forward to it.
On rulesThe rules are a bit awkward right now, and centred across multiple books. The main rulebook (with all the core mechanics of the game) can be found in the main 'The Hobbit: An Unexpected Journey' rulebook, as seen here:-
http://www.games-workshop.com/en-WW/The-Hobbit-An-Unexpected-Journey-ENIt's quite bulky and pricey, but there's a pocket sized version available with the 'Escape from Goblin Town set'. It usually comes up on Ebay for a bit less money. The downside to this is that the larger version also comes with the rules for some Hobbit-centric models/warbands that can't be found anywhere else and aren't included in the pocket sized version. So if you want a complete set of rules, you'll grab both, one for those rules, and one for easy play use.
There's also been fresh Hobbit warband rules released in 'The Desolation of Smaug' expansion:-
http://www.games-workshop.com/en-WW/The-Hobbit-The-Desolation-of-Smaug-ENas well ones from their Five Armies expansion, which are available to download for free here:-
http://www.games-workshop.com/en-WW/The-Hobbit-The-Battle-of-the-Five-Armies-Strategy-Battle-Game-PDFVirtually all of the original LOTR armies on the other handcan be found within one of these five sourcebooks:-
Mordor (which carries rules purely for the Mordor Army List)
Moria and Angmar (Which, unsurprisingly, carries rules for those two factions)
Kingdoms of Men (which contains rules for Rohan, Gondor, the Fiefdoms of Gondor, and Arnor)
The Free Peoples (Which has rules for the Dwarves, High Elves, Wood Elves, White Council, Wanderers in the Wilds, The Fellowship, & The Shire)
and the Fallen Realms (with rules for Isengard, Harad & Umbar, & The Eastern Kingdoms).
Unfortunately, all but the first two are out of print, and the remainder go for high prices on Ebay (up to fifty pounds!) Let me know if you have difficulty sourcing the Kingdoms of Men one though, and I'll see if I can help you out.
On CollectingThe LOTR game was on GW's backburner for quite some time due to poor sales. With the recent revival and a change of heart towards specialist games though, GW is advertising for fresh model designers and rules producers for LOTR literally right now, so we'll probably get a revival of support from the company in a year or so.
Right now though, many models are out of print, and hard to find. There's a number of recasters on Ebay who charge extortionate prices for copies of some figures, so if you're willing to put your hand in your wallet, you can usually find 99% of things, but high demand models that were produced towards the end of the game manufacture can be pricey.
The current ruleset dictates that you need a minimum of one hero for every twelve warriors your have, so you tend to find far more Heroes in armies these days. Gondor is lucky in that they have a pair of super cheap heroes though, that allow them to bulk their forces out quickly without having to spend many points (Damrod and Beregond). So if you're hoping to field Minas Tirith, I'd pick those two up first, along with a large batch of standard Warriors and Knights. If you supplement them with some Citadel Guard with Longbows, some Guards of the Fountain Court (with shields added) and one or two more heroes of your choice, you'll have a flexible and usable army in no time.