When they are talking about Flat Slide it is a type of carb where there is a rectangle box sitting vertically inside the cavity of the carb.
(kind like a sliding door that goes up and down)

when you pull the accelerator cable it is lifted and opens up the 'throat' of the carb letting air in and also lifting the 'needle' which unblocks a jet at the bottom which lets fuel come up and mix with the air.

There is also 'smooth bore' and round type and also semi flat. All have different properties but work on same principle

In fuel injection you dont have slides you have butterfly valves that turn on a shaft the same as how car carbies work.

I think a setup from a gsxr 750 would go well in your application, just work out what hp you expect to make with your A12, then google the stats of different bikes.

They are all pretty close in power so it is ok to just stick to one manufacturer for comparisons.... ie, Suzuki GSXR 600/750/1000 you fill find the yamahas, hondas, kawasakis 600/750/1000's make about the same power for each model of the same size.

As for a computer you could probably go a microtech mt-4 or something similar but i would recommend you get a "microsquirt" they are VERY small compact fuel injection computers (about the size of a wallet) they have sh!tloads of functions, and are cheap.

You can use a microsquirt to control your ignition as well (with a couple of addons such as ignition pickup wheel & sensor etc)

Just keep in mind that the micro squirt does have limitations compared to a mega squirt or any other CAR ECU becasue they are meant for bikes.
For eg. They can only put out enough power to drive one coil, so if you want a 4 coil setup you need an extra device to power the coils.

But all in all, they are suited very well to the project your planning. Probably a bit trickier to set up at first, but a lot cheaper then anything comparable.

have a look at
http://www.microsquirt.info/

I will be using one of these for my bike EFI conversion onto A15.

Hope that helps mate!