I'm looking for one system to run all the basic and advanced modules, you know, stuff like 'The Keep on the Borderlands', and 'The Temple of Elemental Evil'. I also want to use it to run the Pathfinder adventure paths.
Any of the three Microlite74 Version 3.0 editions should work for any of TSR modules with only minor changes needed to the module -- the same type of minor stuff that would be needed to run a Basic D&D module in AD&D for example.
I'm not really familiar with Pathfinder adventure paths, but they should work as long as you are willing to change and adapt things like monsters (use the less complex stat blocks from M74 monster lists instead of the massive 3.x style stuff. Some spells in the modules will probably not exist or will work in different ways than their 3.x versions. Some things 3.x takes for granted are very different in M74. For example, there is no "concentration" skill so any spell-caster hit before his spell is cast loses the spell. This means that certain encounters may need adjustment. That said, M74 is based on M20 which is a very lite version of 3.x which works fairly well with any 3.x style material, so it should be doable without too much extra work.
Is Microlite 74 ext. the best option to run those old school modules?
That question has no objective answer. The best option is generally the rules the module was written for, but that's not a really helpful answer. M74 is regularly used to run TSR-era modules without many issues.
What's Microlite 74 version 3?
That the current version, in three different "editions": Basic, Standard, and Extended. Standard or Extended would be best for most TSR era modules. Extended with some of the optional rules from Companion I would probably be the best for Pathfinder modules.
Sorry for the slow answer, but I've been doing Thanksgiving stuff the last 4-5 days.
