FT 70-300 versus mFT 75-300

Hello,

Is it a good idea to pick up a new FT oly at 319€?  a plan an upgrade from a e420 to an OMD.  I only have de two kit lenses.

How good are the FT kit lenses working with the adaptor?  Idem for the 70- 300?  Is there a real advantage with the  F5.6 at the long end?

If the old lenses are usable without to much trouble, i could skip the 12-50 and buy the 17 and 45 mm...

Thanks for the advice!

steven