Akro,
You need to do at least five hours dedicated to just the ME flying if you are just doing your first ME endo (no I/F) and then when ready to upgrade, fly a few approaches while the instructor fails things turning inbound and going around...adds up to a bit more cash IMO. Don't forget the second test fee.
You can cut that back a bit if you are doing the MECIR as you will be able to hone the circuits and OEI ops during the circling approaches of the CIR after two or three hours of purely ME handling. You should be flying half of the in flight hours in a single anyway. Don't have to do the whole flying component in the twin and you won't need the endorsement signed off until the test anyway.