Aroughton
I've no complaints about the Avanti Trei which is, as you say an open heel fin. It has 3 sections i.e. 2 soft dividers, compared with the 4 of the Quattro i.e. 3 soft dividers. There's no denying that the Quattro is rated as one of the best if not the best blade fin on the market but the price comparison in the UK is £40 against £80. I don't think the Trei is too stiff. My wife has the Trei in a small size and she thinks they are very good fins. We use our fins for Rec diving only so high performance isn't an issue. manoueverability is what you need.
As for what to get now I would suggest you get mask/schnorkel, fins, boots and wetsuit. These are all personal items which need to fit you. I would strongly recommend open heel fins with boots. Boots means that for shore diving your feet are protected both on the sand/rocks when approaching the water and when you walk in the water. You then put your fins on when your chest deep and you have your BC partially inlated. The Mares fins have an outstanding clip/clasp type fastener which makes them so easy to put on and take off.
I'd recommend a full wetsuit of whatever thickness is appropriate to where you will be diving. For example I have a 3mm wetsuit for when we warm water dive and a 6.5mm semi dry for colder waters. You often see people on this board ask if they need a wetsuit in very warm water i.e. 78-82f. Technically no for just one dive but for repetitive diving, say 2 or 3 dives in a day, you need to retain body heat. Even 80f water is 18f cooler than your body heat. You never over heat underwater and you'll notice the difference even in a warm indoor pool after you've been in for an hour doing your confined water training.. As for why a full wetsuit even for schnorkelling, the answer is that you are protected from stinging critters. Those who wear shorties are not!
I think you're wise not to buy the rest of the gear until you know that you're hooked (you probably already are but haven't realised it yet - it's a nice addiction to have!) and more importantly you know what diving you want to concentrate on.
As for buying from an LDS rather than buying over the net I do both. I get the best online price and, like Tavi, see if the LDS is prepared to come close to it. If they are I'll buy from them. However, I've come across more than one LDS who just quote MRSP and think I ought to be grateful for the opportunity to buy from them. No thank you! However, I'm prepared to pay something of a premium over the online price for good service.
Don't feel guilty about shopping around - you worked hard for your money so why should you give it away.
Best of luck with your course.
regards