Others will have more informed opinions on your broad questions, but on the BP stereo question, you can get an OEM fascia/AC control panel that allows a double din stereo to be installed, they are just hideously expensive asthey only came on certain spec BPs so they are like hens teeth....
https://www.trademe.co.nz/motors/car-stereos/wiring-kits/auction-1441113260.htm
You also occasionally find one here that already has that kit
https://www.trademe.co.nz/Browse/Listing.aspx?id=1366150725
And I think the EZ30/36 are higher compression engines so wouldn't run well on 91? though others will know more about this than me... so this would lead you to 2.0/2.5 NA?