Fitting a toilet into main bathroom

[elduderino]

Hi all, I was hoping for some advice and ideally a ballpark costing:

We live in a ground floor flat conversion c1900, currently in the flat is an existing seperate toilet and a smallish bathroom with no toilet. Both are on the same side of the flat, against the outer wall and appear to feed into a sewer pipe running down the side path (our main path) of the house.

We are looking to renovate the existing bathroom and would ideally like to fit a toilet within the bathroom and maybe block the old one off creating a cloakroom.

I'd like your advice on the scale and practically of the job and also how we go about getting quotes (do we simply as plumber/builder to quote?).

My existing reading leads me to believe we need to create a new soil pipe, probably through the wall and try and join the sewer downwards?

Happy to try and supply any more information that would help..?
Many Thanks, Simon