Regarding the use of a simpler/cheaper twin for the rating, that probably depends on what you want to do with the rating. One should also consider aircraft checkout and insurance requirements. My insurance policy, for example, requires 10 hrs dual in type. So even if you have a ME rating you would still need 10 hrs dual in the Seneca if you wanted to take it out solo. In light of restrictions like this it may make sense to get your ME rating in the aircraft type you plan to rent or buy.