As the first step, I'd have a look a the kind of warband you want to build - the theme or style that most appeals to you. More fantasy, or more historical, and if the latter, what kind of period?
Quite some options are covered with a bunch of plastic sets, which is good as you probably want some variety in the amount of armour and weapons so that you can differentiate the troop types. The actual Frostgrave plastics obviously have most of what you want; they tend to be light on armour, but you can sculpt some chainmail from under the robes, or use some of your Crusader parts for example.
I've previously made a Renaissance Italian warband using Perry late medieval plastics (pretty much all the bits you could wish for), another Norse-themed one using Gripping Beast plastics (armoured Vikings and the unarmoured dark age guys, simply not using any crossbows which isn't that big of a disadvantage), and yet another one from the Warlord Games Celts (armour and unarmoured on one sprue, mostly melee focussed but I added some javelineers which are troops from one of the supplements).
Various manufacturers combine fairly well, but within a single range the compatability of scale and parts in general is typically very good. Victrix also has some nice kits with mixed armour and weapons (Germanics and Vikings come to mind), and you could go for the Oathmark sets too (including Elves and Dwarves - they may have no specific rules in Frostgrave, but precisely because of that you can use whatever you feel like!).
It may be worth looking for companies selling individual sprues (there's a good ebay seller for these too by the way) so that you're not stuck with 60 armoured troops, 60 unarmoured ones and 32 archers or so. I'd try to find a range that covers most of the things you want (two-handed weapons, armour, shields, bows and/or crossbows), but you don't necessarily need to have every option available. Daggers are easy to make, as are staffs from cut-down spears or so. You can even think of your wizards while deciding what troops to build - my Celts for example were of course led by druids (represented by the Witch school) who used their magic to create a lot of dense fog on the table, meaning I didn't need/want long-range weapons anyway, so it was fine that I didn't have suitable plastics for archers/crossbowmen.