Over 10% you have to tune for it. I really don't care what people on the internet tell you, you will have to maximize your gains by having the car mapped or running one of the platforms that support map switching and an eth analyser. You will have to upgrade your injector capacity (or add port injection) and other parts of the fuel system beyond a certain point on majority of cars. This will have an impact on your regular use of the car because you will either have to switch maps (and not all cars can do this easily) or constantly run ethanol... which at high concentrations runs out QUICKLY. I could get 280km out of a tank of petrol and 100... MAYBE... with E85.
You also don't know how much ethanol is in the fuel already. It is definitely added but how much remains a mystery. Sasol says 'no more than 2%' but you can have 'up to' 15% legally.
What car do you want to use it on and what do you want to achieve? If it is a car on plan, FYI BMW requests your downpipes and a sample of fuel for motorplan claims these days.
There is no specific brand of Eth... Eth of a certain purity/concentration is what it is. the Eth based octane boosters that have some lubricant and colour added with maybe a few other VOCs have their fans which I won't get into.
I used to have TAG solvents deliver 220l to my house. There are minor by-law issues to get around like having ventilation and some no smoking signs. There's also a limit in terms of litres you can store on site in a residential area. It counts as a Class 1 flammable liquid so you can read up on that. That being said, for 25l it is immaterial. With the 220l, though it was going quickly as we were running 85% in my old Subaru and 10% in my F10, there'd also be a distinct change in smell in a week or two... If you've ever done chemistry you'll recognise it... related to it clawing moisture from air and decomposing. There is just too much of air that stays in the vessel despite being air tight beyond a certain point. Still works fine for the most part if you use it quickly but you can't buy 220l and store it for months on end. This is also one of the reasons modern fuel 'goes bad' more quickly - eth content. It was happening to the degree that I started selling it to get rid of the 220l more quickly. Since I had no desire for this to become my day job, I just switched to aforementioned actual high octane petrol from 24/7 - you then have a car that won't be pulling timing as it would with pump fuel and you won't really have to tune for it.
Bear in mind also STORING ethanol is a mess, even when mixed - it expands and contracts dramatically with temperature so get used to having containers looking like they are about to burst, the smell and of course the occasional burst in your early days of use. When you are using higher concentrations than an 'easy' 10%, it is a 'way of life' that quickly becomes tiresome to some (of which I am one). You can buy eth analysers etc for many cars that make this easier and will adjust your map as well if tuned that way.
No ambiguity whatsoever that way.
