Model wise, if you want good products for your money, look at the Perry brothers, stuff like their war of rose models are very nice for an Empire army and you will get something like 40 models for the price that would get 10 with GW.
Quality wise, the Perry brothers used to work for GW and were among their best scultors, the only potential problem is that their historical range use a more realistic scale compared to GW "heroic" scale (big hands and weapons mostly), so you will usually not want to mix models from both in the same unit, but otherwise it's perfectly fine to use both in the same army if you like them.
All the following boxes are £20.00 :
(for heroes or war engines, they also have a metal range with some decent options, at least for normal humans)
English Army 1415-1429
The box contains: 36 figures, unit bases, 17 banners, 6 standards, painting guide and information sheet.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=3217Agincourt French Infantry 1415-29
The set consists of 36 Infantry and 6 knights/men at arms. The infantry are armed with spears, crossbows, double handed axes and hand weapons. The knights have cut-down lances/spears, pole arms and swords.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=3414Plastic Wars of the Roses Infantry 1450-1500, bows and bills (40 figures).
You can build up to 30 bowmen, up to 18 billmen and includes 4 fully armoured command.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=2471Agincourt Foot Knights
This box contains 36 Knights/Men at Arms. There 18 'English' knights and 18 'French', however all could be used for the same side or any west european nation. They are armed with cut-down lances/spears, pole arms and swords.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=3415Foot Knights 1450-1500.
The box contains 38 multi-part plastic figures, 17 flags, unit bases and infomation sheet.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=3148Mounted Men at Arms 1450-1500
This box contains 12 mounted Men at Arms that can be used for most European armies. There is seperate horse armour for mounts if needed and 7 different horse heads (25 in total). The Men at Arms have 14 right arm options and 12 head options (per 4 riders). There are also lance,sword,mace, warhammer,axe and standard pole/spear options.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=2713Light Cavalry
The box contains 12 plastic horsemen that can be assembled with light lances, crossbows or as mounted archers. It also contains parts for command figures, banners and historical infomation.
https://www.perry-miniatures.com/product_info.php?cPath=22_62&products_id=3275Finally for things like Fantasy heroes and wizards, and big monsters, one of the best option to get models for cheap is Reaper, especially the models made in their soft "Bones" plastic (search "Bones" in the shop), but evne their metals are decently priced :
http://www.reapermini.com/Rules wise, my favorite for ranked mass battles is Kings of War, but of course it can depend upon your personnal tastes and of what is already played in your area (but of course there is also the option of starting your own group of players)
One thing to remember :unless you plan on mainly playing in a GW store or play official tournaments that impose models from them, don't feel that the rule system that you use force you to use models from the same company.
Outside the specific cases mentionned above, it's perfectly fine to play Warhammer with Perry models or Kings of War with Games Workshiop models.
(In fact, official KoW tournaments don't even restrict you to their models, except for the "best painted army" award that need 90%+ models from them, but this is secondary)
Same with the background : there are many former Warhammer players that didn't like what Age of Sigmar did to the setting and the game that switched to Kings of War and still use their original army with their fluff and background unchanged, KoW has a large enough range of generic fantasy themes (plus some specific options made to facilitate the transition from Warhammer), so almost any Warhammer model or unit can be used in KoW.
I will stop there for now, don't hesitate to ask if you want more information about KoW.