its nearly exactly the same M103 engine used on all 300Es, although the 90-92's had some very minor tweakage. note that 1993 300E's are actually 3.2L with the 104.992 engine, which is very different, with electronic injection rather than CIS-E.
sure, but they still have to be upright. AGM batts can be used in any orientation... I'm thinking that instead of 1 80AH car batt, you could use a pair of half size batts on their side so they are lower profile and don't stick above the deck, like a couple motorcycle batteries.You can get ventable batteries in the interior, they use a tube to vent to the outside