Advice on buying a CX bike

Weight - I've seen a handful of bikes (generally with single chainring) as low as 7kg, my XLS is about 7.5kg (double chainring, not standard build), but that's very much the lighter end of the spectrum. Anything in the 8-9kg range should be fine. You're going to end up flicking it over barriers and lifting it onto your shoulder a lot, aswell as lots of accelerating out of corners, so light is good.
Brakes - disc definitely better, but cantis are fine; the better you get at 'cross, the less you need the brakes. That said, if you get hooked on the sport you may end up with a load of wheels, which means you'll be stuck with whatever you decide now (I suspect this is one reason more people haven't switched to discs). Discs are only going to get lighter/better. But at the end of the day either are going to be fine.
Tyres - You can get most of the benefit of tubs with a fraction of the cost/hassle, with tubeless. If you're serious enough for tubs to be worthwhile, you're probably going to want at least two sets (with different treads), plus a set of clincher wheels for training. Why not start off on clinchers or tubeless, and if you get keen and move onto tubs you've got the clincher wheels for training. I doubt any amateurs train on tubs. Most of the field will be racing on tubeless or clinchers at local league level, it's only really at National level that the majority of riders are on tubs. Tubs are undoubtedly better (I'm switching this year) but probably not worth it until you've done a bit of racing; to begin with, technique is going to be more important than tyre choice.
Bikes - I really rate the XLS as a race bike; I bought the frameset and built it up myself. Watch out for cheap deals on eBay, I just bought another XLS frameset for £400, because the paintwork had a couple of chips. Kinesis Pro6 is also popular, almost identical geometry, and takes mudguards etc (I have one built up as a commuter, though I'm sure it'll see a race course at some stage).

A bit of perspective on some of the disc brakes I've used:

Started with BB7s, so these are my benchmark. Seem to work pretty well, easy enough to set up. Require periodic servicing (total strip down and rebuild); if you don't do this, the pad adjusters sieze and stop working. Need to adjust both pads separately as they wear.

Switched to TRP Spyre, with a new bike build. Quite a bit lighter, pads don't tend to rub as much. Once set up, can adjust for pad wear using the cable adjuster. Hard to imagine a better cable disc brake.

In both cases, cable contamination was an issue, even with supposedly sealed cables. In muddy conditions the cables were getting noticeably sticky after a month or so (because of the cable routing, I suspect this is more likely to be a much bigger issue with discs than cantis). Like most cross bikes, mine get pressure-washed pretty frequently, and I'm sure this was a contributory factor.

Switched again to Parabox (full hydraulic), mostly because I found some cheap ex-demo sets. Setup process is pretty time consuming and fiddly, but once installed they are virtually fit and forget. No need to adjust for pad wear, but the real eye-opener is the fact that there's been no degradation in performance because they hydraulic system is completely sealed; despite zero maintenance they're as smooth as the day I fitted them. For this reason, I even fitted them to my commuter!

I'm not suggesting you fit hydraulics, but for CX disc brakes they're definitely the future.

Note that most brakes (definitely the Spyre and Parabox) come with organic pads, which get really badly eaten in some conditions. On one occasion mine went from brand new to completely ineffective in about 30 minutes; on the positive side, I learned a lot about how to get round a course without using the brakes. Sintered pads last way better, and any difference in performance is irrelevant for cross.

I did make a real effort to keep the weight of mine down; most components are carbon, and I've just accepted that I may have to repair/replace them a bit more often; I do know how to repair carbon components, which is a bit of an advantage, and I kept the costs sensible by buying stuff 2nd-hand and looking out for special offers. Couple of bits I doubled up on when there were special offers, in anticipation of 2nd bike.
Wheels: Hand built (by me), Novatec 711/712 hubs, Stans Iron Cross rims, DT Comp spokes on high tension side, DT Revo spokes on low tension side. ~1400g total
Rotors: KCNC Razor
Tyres: Mostly Clement tubeless
Seatpost: Lightweight carbon
Stem: Cheap alloy, 130g
Bars: FSA carbon (special offer)
Shifters/mechs: SRAM Rival
Chainset: FSA SLK Lite
Brakes: TRP Parabox
Frameset: Planet X 1.9kg
Pedals: Time ATAC XS

The weight is obviously less of a benefit when there's another 8kg of mud stuck to the bike, but for the drier races it has to be a benefit.

Tyre pressure: I've raced tubeless at 19psi (on purpose) and 17psi (by accident), and the extra few psi really make a difference. The only real issue I've had with tubless is burps, and that (and the prospect of even more supple sidewalls) is what's luring me to tubs next year. Having TT'd on tubs in the past and sworn never to touch another one again, I'm not looking forward to all the faffing. I still TT on clinchers...