if possible repair or replace the 512k trapdoor ram add on, mod for 1mb chip ram (if the req agnus version), upgrade to kick 3.1 and add an 8mb+ide cpu socket card for a small performance upgrade (real fast ram)
then you can run everything from a bootable workbench 3.1 setup via WHDLoad with the bonus of being able to run applications with 8mb of fast ram.
or save some money
do the same 1mb chip ram upgrade iff possible and replace internal floppy for a gotek and run everything from USB stick (slow loading like a real floppy and no performance increase).
other than the vampire (which will enable AGA , 2mb chip ram , hdmi etc) the "best" is probably a cpu socket upgrade 030 built new here on amibay or a real GVP external sidecar 030+HDD but hard to find.
but i do recommend the ACA500+ (plus version) comes with 8mb fast ram, 2 CF card slots, mapable roms switch between 1.3 - 3.1 or custom roms like 3.1.4, action replay and a CPU boost upto 42mhz but stable at 28mhz. and if possible (depending on your a500 version) it will map your 512k trapdoor ram as chip ram so no mods required.