Looks like a pretty boy... esp for a TB!
Don't know where abouts you're from, but I see gum trees & the word 'lucerne...
$350 for a 5yo horse who is going under saddle & has no health/soundness issues(windsucking not withstanding) is a pretty good price in this neck of the woods. Even for a TB
Remember though, the cost of the horse is generally the cheapest part of owning one though. Aside from feed, agistment if you don't have your own property, there's regular farriery, equipment, the regular vet's stuff like shots, teeth, etc, as well as putting money away for an emergency fund.
I don't believe windsucking is due to boredom or such, but is due to gut probs, as you mention, ulcers for eg. Likely developed from his time as a racer. So I'd treat for ulcers, feed probios, extra magnesium, and ensure he's always got hay/grass. I'd also keep to low carb feed if you feel he needs extra - grain & molasses free for eg.
I'm a great believer in the importance of good nutrition for health & wellbeing. This starts with educating yourself about equine digestion & nutrition. It is a huge & often confusing subject though, so I suggest FeedXL.com as one good (Australian) program/service that can help take a lot of the confusion out of balancing diets & working out what prods/feeds may be most appropriate for your horse.
If he has adequate grazing, what's the one biscuit of hay for? Is this lucerne, for it's different nutrients, or is the grass too rich so you want him to have some low sugar hay to 'water it down' a little?
Why will you be hard feeding him? Why only a few times a week? If you're wanting him to gain weight(doesn't look like he particularly needs it), or think he needs more energy because he'll be in hard work, then you really need to feed at least once or twice daily, and especially if you can't feed at least that often, keep to ultra digestible & low carb feed. And small meals - only around 2-3kg max. If you're feeding for nutritional supplementation, then while it's still best to feed little & often, he will still get a fair bit of benefit if it's only every other day IMO.
Why lupins? Is it for extra energy, or nutritional content, that they will balance out what else he'll be getting? They're high in phosphorus & omega 6, among other stuff which could be problematic, depending on what else he's getting(for eg. horses usually get enough o6 in their diet, but short of o3, and these 2 need to be balanced). There's something else about legumes, inc clover, that's slipped my mind ATM(something about hormones maybe), that hopefully someone else will elaborate on.
Chaff; I'd avoid wheaten, but either of the others may be appropriate. Oaten can be high NSC(esp if it's got any oats in it), not too much in the way of nutrition though, so more a filler with extra energy. Lucerne is low sugar, high in energy, protein(so are lupins & showtorque), calcium, potassium & other stuff, which can all be good or bad(well, cept low sugar never bad IMO), depending on what the horse needs.
Showtorque; Is he going to be in hard work soon, that you feel the need for this sort of thing? If not, wouldn't bother. If so, I would personally choose a non-sweetfeed, high quality alternative, and pref. something that's low protein & o6, without the added legumes, esp if he's getting those elsewhere too, in lupins & lucerne. I'd also choose something that's a better 'ration balancer' than this, that will provide him with the nutrients he's missing in the rest of his diet. Or else add other nutritional supplementation. I also wouldn't feed something like this if I couldn't feed at least 2 small meals daily.
Sunflower seeds are also high in o6 fatty acid, so given the rest especially, and that he's got grass to graze, wouldn't be feeling the need for more of that. Alternatively, flax/linseed is rich in o3, so would be an appropriate additive. **Vegetable oils are fragile & go off quickly, lose their nutrients & turn to transfats with heat processing, so I buy whole linseed & grind it daily for feed. If you buy oil, make sure it's cold pressed & stored in a lightproof container in the cool.
Don't know what the celery seeds are for?