Your Vertex ( I presume Mk.2 2JZ-GTE Mk.iv engine) will take up to 450hp on stock turbos. Although longevity is debatable. That is a boost level of 1.2bar.
If you do go for a kit, you want an HKS type which is rated to 550hp. You want the kit because otherwise the wastegate etc. is not correct. Just changing the turbo won't be as efficient.
Remember at 550hp, your turbo will be a bit laggy especially with the auto-box. I would not get a turbo rated to above 550hp with the auto box.
Downsides - you'll wear out your gearbox faster as it's only rated to a reliable 400hp. You'll need fuel pumps, bigger injectors, fuel controller and boost controller if you haven't got one. You'll also need a medium sized intercooler too (run the stock intercooler and you will kill your new 550hp rated turbo).
Your power band will start most probably around 4500rpm and peaking at 6500rpm. That means it's all-or-nothing power delivery.
Erm the power delivery on a T61 isn't that bad, mine starts coming in at about 3000-ish and is on-song good and proper from 4000 rpm. I agree about the box though on a mk1 dunno if the 2 is a different box. I have to say with an auto box you will not get as much as you would hope for out of a single turbo. I would suggest that a fuel pump that will handle BPU ie walbro will also be OK for a smaller single, but yes a bigger set of injectors ie 650cc will be required and a way of controlling them also (this is where stuff gets complicated and expensive and for that matter problematic).
You will obviously need an aftermarket manifold and I am not sure the mk4 supra one bolts straight on but you have a choice here again of a cast one (guy called arnout in holland makes these) or a welded tube (which is significantly cheaper), either type though will have provision for bolting on a wastegate (I am hearing good things about Tial now but again there is a wide variety of budget options here).
Intercooling you will require an upgrade yes, Chris wilson a guy on the mk4 forum manufactures uprated SMIC's for the mk4 and I am sure he could modify one for the aristo (these are rated to above 500 horses so more than adequate for a small single and you can keep the stock airbox which is a very good thing)
The stock tubbies however run all day and all night at 1.2 bar BPU over on the supra forums in both the UK and USA and from someone that squeezed all there was from a set of stock tubbies and now has a small single I would say stick with the stockers, fit water injection and higher lift cams to squeeze the most you can from the car and leave it at that. Time off the road and constant hassle with tuning and fixing parts that can't take the power is a real ball ache for a car that you just can't legally use the power of.