Well with a motor of that condition, dodgy head unknown head condition and most likely whoever did the head work retorqued it to 55ft lbs
i would certantly expect a BHG sometime in the future..
For a metal head gasket you need to have the head and block perfactly flat and have a very smooth RA surface, so in other words you need to do a full rebuild again, you cant just put a metal head gasket down
My 7m blew a headgasket a year and a half ago, so i had the head plained and put a geneuin toyota gasket down with ARP studs and shes still going strong with no sign of BHG
You could just do this if your nervous about a BHG
As for 2nd hand heads i wouldnt trust it pretty much every 7m out there has a BHG who knows how much has been plained off the head, warpage etc, new heads from toyota are around $900
That intercooler will fit fine in the front of your supra but not alot of it will get airflow...most of the cooler will behind the bumper and bumper support
7M gear for the R154 is rare and expensive, but if you have the $ it can be done, clutches are also very expensive. tracking down a manual pedal box is getting hard for the mk3
check out
www.supramania.com they will answer alot of your questions as the amercians are doing this all the time