If budget and bang-for-buck is a consideration, I vote for a twin turbo'ed flat six Subaru engine or a 4-cyl. STI. Not new territory in mid-engine conversions.
Engines are plentiful, relatively light, low cost and seem to be fairly strong. After experimenting with boost pressures, you will appreciated the low cost.
Subaru mid engine swap?
subaru mid engine conversion kit - Subaru Impreza GC8 & RS Forum & Community: RS25.com