Just send me your credit card details and I'll sort it out
Only joking mate. The AE tank is a nice bit of kit. I'm debating on it being my next setup. As for co2, co2 Art and all of our sponsors have some decent gear and good customer service because they often get scrutinised in this forum and a lot of people here buy kit off them. Only suggestion I would have here would be to get one with DIN477 thread so you can use fire extinguishers or pub bottles. The 600 or 900 @ 200 ltrs is going to get through some gas. Disposable or branded gas bottles can work out expensive or hard to get hold of depending on where you live. Most people can get hold of a fire extinguisher or have a gas dealer locally that will refill pub bottles.
The lighting, worlds your oyster really. Your best best is to set what you can afford and look for stuff in that range. As you've found out the Twinstar and Kessil stuff comes at a premium. Most leds look sexy these days and regardless they also form part of the tank aesthetic. If I'm going to be staring at something all day I'd like it to look nice and premium on the tank but underneath it light is light. Some lights render different colours which can change the whole effect of the tank, some have a slight pinkish hue and some make greens stand out. People generally go for lights around 6500k which looks more natural and makes the greens stand out. The majority of planted tank leds usually are around this colour anyway. Lot of people here use the Chihiros ones which are a bit more reasonably priced. Use the forum, find a light you like and search the board for people's opinions. If you can't find an opinion make a post, someone will have owned one and tell you the pros and cons. Whatever you get just make sure you can dim it if necessary.
Substrate, me personally, I look at tanks like houses and the substrate is the foundations. If I was going to drop the neck end of £1000.00 on equipment which it looks like you're about to do I wouldn't skimp on the substrate. I've seen people mix and try various things with underneath layers of this with that on top and had issues with stuff mixing and clouding water when uprooting plants or whatever. I play it safe and go with a brand, once it's in it last for years and can be reused in most cases. However, I have a stash of cat litter that I use for various short term projects that I keep in a 25ltr bucket and just keep washing and using again without issues. I think I bought about three bags of it for about £12. On the other end of the spectrum for my living room looking at it all the tank scape I used Tropica Soil which cost around £70-80 I wouldn't mess with substrates unless you know what you're doing, ultimately if you get it wrong you're ripping the whole lot out again.
Filters, again, the worlds your oyster. They suggest looking for one that will turnover 10x the volume of the tank as when loaded with media it will be lot lower in real world on the 600 aim for over 1000ltrs per hour on the 900 you want to be around 2000 ltrs per hours. You can always add extra pumps to move water if needs be but that's just more equipment in the tank which ruins the overall look so try and get as much turnover as you can and check in the forum for other people who might have one and see what their experience is with it.
Other than that happy shopping, I can smell the plastic burning from here
only other suggestion I could make is some of our sponsors when you register with them will send out codes for 10% off or similar as well as other online retailers. Register with them and see if you can get as much as you can from one supplier, stick them all in your basket and buy a few bits and pieces of inexpensive stuff now and again, diffuser, drop checkers them sorts of things. Wait to see if you get a coupon or discount then drop the bomb on all the big items. Also saves on delivery costs.
That's what I thought, just thought it was worth checking in case somebody out there on ebay or something was claiming to be you and your own brand of substrate. I do think it is important that people still buy commercial products to keep the industry alive and visit their LFS but in the OP's case the ferts were going to get really expensive for them.