Rules, well I’m a big battle kind of guy, so Great War Spearhead or Square Bashing (both use 1 base = 1 company so armies are interchangeable).
For a simpler game, the One HOur Wargames WW1 rules (Machine Age, with the tanks and barrages optional rules) actually work really well. There is a WW1 mini campaign on the Neil Thomas yahoo group (the group is called AMW afterhsi Ancient and Medieval sets) we’ve played through.
I also have tons of 20mm WW1 figures, mainly plastic. Plastic Soldier Review is your friend, but Emhar, Revell, Esci, HaT and even venerable Airfix do great figures. My cavalry are mainly metal (IT Miniatures, Tumbling Dice? I can’t recall). Emhar and HaT do piles of cheap, good quality plastic artillery. You’ll need lots of guns. Irregulars ‘Really Useful Guns’ range is great for cheap heavy artillery (6″ guns, Krupps 150mm, Schneider 155s etc).
You won’t need loads of tanks, but there are plenty out there. Mine are mainly Emhar and Airfix (Mark IVs, Whippets, a lonely A7V etc), my FT-17s are Armourfast. The French are less well catered for but Lancer do a nice St Chamond. My Schneider CA is a card model, which was really horrible to build, but it was a horrible tank:)
Because I’m lazy, my planes either second hand ‘Wings of War’ prepaints, or some old diecast models I bought years ago.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke