There’s a couple of good schools for this, starspeed are one I’d recommend - they should give you for info on prices types etc.
Insofar as SEIR versus MEIR, some offshore operators now don’t require a multi engine IR, and will consider a single engine. Most new entrants on the s92 or 175 are doing a heavy TR when they join so the conversation isn’t that big a deal. However, if you’re up against another applicant with an MEIR and are matched in all other respects guess who’ll get hired 😉
I don’t really know much about the onshore market, but most companies advertising jobs will likely not respond if you don’t have the minimum requirements - it’s a competitive employment marketplace for aviators at the moment.
As always research is everything, best advice is to call all the companies you’d like to work for (chat for a bit first) and ask them whether they’ll cover a single to multi conversion as part of line training. Also, gets your name in the mix for any future positions.
Hope that helps and very best I’d luck to you.