Does 3.5E have standard stats for Bandits, Soldiers, etc? If so use those, while tweaking the weapons they use to add some variety.
T1-4 is 1E AD&D, not 2.5E, and is an ambitious project to attempt to convert, especially if you are not familiar with 1E presentation. The statistics provided for the DM by the adventure were very bare bones, and assumed the DM to either have a Monster Manual to hand during play, or an extremely good memory for damage/THAC0/special abilities etc. It was a very poor adventure module in this respect - all of the 'Supermodules' were - they were lazily edited and expected a lot of work from the DM.
T1-4 is also the sort of module that can turn into one long slog if the DM is inexperienced due to its sheer size and the numbers of enemies - it's hard work to run. I really would not recommend it as your 1st conversion project. Or rather - just focus on T1 (Hommlet and the Moathouse), forget the rest. The Village of Hommlet is an excellent base to use for your party, and the Moathouse is a tidy little dungeon and a lot of fun. The Temple/Nodes/etc were um, well they were... erm... a 'disappointment'. They are massive, and took a very long time to play through using 1E rules, with 3.5E running considerably more slowly than 1E due to the extra combat mechanics, this might only exaggerate the problems those levels had.