The toilet has a 3" drain pipe (minimum) and you need a 2" pipe for your washer, so that will work. However, toilets are more or less self venting, since the bowl and tank admits air to the system. That means the vent line may be quite some distance away. Your new washer drain needs a vent that runs to the roof. If you installed the washer drain in the wall behind the (removed) toilet, it should be relatively easy to connect into the shower vent if there is attic space above.

You will need to demo quite a bit of the floor to get access for the plumber, and also demo the wall behind the toilet to install the new drain, vent and P-trap for the washer. Then you have to get an electrician to run power for the combo unit (maybe new 240 volt). Since I have no idea of your local labor rate and the details of your house construction, I would probably estimate $5,000-$10,000, plus the washer-dryer unit. Its not a really difficult job, but requires a lot of trades (demo team, plumber, electrician, sheetrock, trim carpentry, floor installation, etc.).

Bruce

Just adding that a ventless dryer can be installed anywhere. I had a stacked Bosch unit installed in my closet that abutted master bath. It was tied into the drains and water supply from the bathroom.

I live in a building in which many of the owners retrofit stackable units when they remodel so I don't think it is much of an issue. Most typically they are installed somewhere in the bathroom and tie into the sink drain and use water supplied to the sinks.