Arclight - I wouldn't let this conversion project be the deciding factor on which boat to go with - the project isn't that difficult in the grand scheme of things. Dave M. gives a good pictoral overview of the project on his Swancruise website and has mentioned the most challenging part in his response above - not too bad.
The overall costs of the upgrade should be considered when comparing the boat you have in mind to others that are already cutter rigged of course. It's not too expensive if you plan to do the installation work yourself.
One thing I'm a little unclear of though is why the mast head would need to be modified (unless the thought is to rig a Solent stay, which would be unusual for these boats). Unless the boat is very new, it probably has a LeFiel mast, which may have an attachment point already welded onto the front of the mast, about 2/3rds of the way up. You may want to confirm whether this is the case. If so, you just need the double jaw toggle (etc.) for connecting the wire.
If you decide to go with a hanked on staysail (instead of a roller furler), you'll need to decide how you want to attach the deck end of the stay to the tack plate. Ideally, it would be removable in some way so you can sail the boat as a sloop for coastal sailing (it'll make tacking easier, etc.). I chose a Highfield lever, but see also Dave's approach on his Swancruise web site.
You may also want to consider mechanical terminals on the wire (e.g., Sta-loks, Hi Mod, etc.) instead of swagged terminals.