Basement bathroom with septic system

Hello.
We've got a ranch with full basement, house is 10 years old.
Previous owner had house built.
We have a septic system - two tanks outside with typical drainage field.
In the basement is what looks like a sump pump, but it's the grinder / pump setup to pump up water and waste from plumbing under the basement floor.
This grinder / pump set up is already plumbed to a separate room in the basement set up for a future bathroom. There is a black, plastic, vertical pipe coming up from the concrete basement floor in the future bathroom for the toilet setup. It's about 2 foot higher than the floor.
It seems to be a bigger diameter than 4" although I forgot to measure it. It looks to be 5 or 6" diameter at least.
Anyway, my question is this.
In order to put a toilet and sink in this area and make it a functioning bathroom, it seems like the hard part is done. Am I right in thinking that my first step is to put whatever flooring material down, then cut the black plastic toilet pipe and glue on one of the toilet flanges I see at building supply stores, then simply install the toilet?
By the way, there's also a smaller diameter drain coming out of the floor for a sink, and the two water supply lines are in the rafters/ceiling of this room with shutoff valves already set up.

The pool table is installed in the next room over, with full size beer fridge. Seems the right priority at this time is to get that bathroom done!

Thanks for any help or ideas you have on this.