I think it depends where the vehicle would be built and where the primary markets will be. You would want to choose an engine that is commonly available and has a good network of parts suppliers in these areas.
Toyota's 3.5L V6 is a good option. It has good power to weight, it's ridiculously reliable and it's available everywhere in the world. Not much aftermarket tuning potential but still Lotus chose it for the Evora.
Ford's 2L 4cyl Ecoboost engine (from Focus ST) has heaps tuning potential and fairly good package space. Also available quite widely around the world.
Another consideration may be choosing an engine configuration that can be replaced easily. ie you could switch your design from an inline 4 to another inline 4 if a better/newer engine becomes available or if you lose supply of the old engine. A V6 likewise. The WRX engine is fantastic, but due to it's layout may require major redesign if you need to change to another engine later.
From what i can gather from this is that i should go for an inline 4 as it generally would be generally lot easier to use. However I am still a bit unsure which one would be the best what should I be looking for as the defining features for a good engine such as HP and cc.
go for honda k-series, for a mid engined car (i assume it would be a custom framed car) you can build the frame to pickup on the factory subframe mount locations, that way you can use the entire drive train from a donor car just mounted at the back powering the rear wheels instead of the front