More practically, I'd say start by looking around a wizards, pick a model you like, and then work out what class fits him. I got this guy for cheap off amazon https://www.amazon.com/Stonehaven-Halfling-Wizard-Miniature-Figure/dp/B06XXPRCZ4 to use as an elementalist, but you could paint the flames green and give him dark clothes and boom, necromancer. That'll give you a more fun campaign than starting from stats :)
I ordered mine off Amazon a week ago, looks like they still have stock and the price is now slightly lower than when I bought mine: Frostgrave corebook on Amazon
Others have more detailed advice for a lot of things, but the rulebook has a suggestion I like that I haven't seen repeated often, and that's to get a set of children's blocks to use as terrain if you don't have a bunch of terrain from other games. You can stack these into all sorts of shapes for structures, ruins, towers, etc and vary it up from game to game.
I love using a thin tape measure. It works, has a lock, and is skinny enough to get between terrain and minis.
https://www.amazon.com/gp/product/B008AGWOXW/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
I personally bought two, so each side could have their own.
All this said, my campaign mates are pretty chill with measuring. We're all fine with holding it above tense action and eyeballing.
here's where I found the ones I used in my build: https://www.amazon.com/Piece-Army-Skeleton-Warriors-Ready/dp/B00IDBZPIU/ref=sr\_1\_1?dchild=1&keywords=skeleton+army&qid=1620776010&sr=8-1
Depends on if you want to invest a lot of money into the game. Heck you could be super cheap and go with this (or something similiar). https://www.amazon.com/SCS-Direct-Fantasy-Creatures-Playset/dp/B0746TKNSL/ref=sr_1_6?dchild=1&keywords=fantasy+miniatures&qid=1610148920&sr=8-6
I scrapped some dudes from Conan the board game. I've seen a guy who plays Frostgrave with lego mini figures. Go to your local game store, and pick up a handful of Repeer bones minis you dig.
I picked up a 4'x4' gaming mat then started with these
Pegasus Hobbies Gothic City Building Ruins Set 1
Pegasus Hobby WWII Gothic City Building Large Set
Those ruins are cheap, look great and easy to setup and breakdown
The sci-fi parallel to this could be based around Stalker computer game. Or more precisely Roadside Picnic novel: https://www.amazon.com/Roadside-Picnic-Rediscovered-Classics-Strugatsky/dp/1613743416
So your sci-fi warband have the leading Stalker commander with different set of alien artifacts.
I just bought 3 different Pegasus Gothic Building kits on Amazon. They are lovely! None of them are pre-painted. They are very good quality injection molded, heavy, gray plastic. The pieces snap together, but should be glued with plastic cement. There are small seams and gaps between the pieces which may need to be filled with putty.
All 3 kits are compatible with each other. Here's the largest kit which looks like it will build at least 4 large buildings: