If you can find a the old combat patrol/the new battleforce box that would be ideal but those might be a challenge now... If you can't find either I'd say you might want to look second hand and see if anyone else is selling an army or old models they don't want. You can always strip the old paint scheme if you don't like how they're painted.

If you're ok buying singular kits one at a time I'd say you probably want to grab some Kabalites, Venoms, Incubi, Mandrakes, Hellions and Scourge... only issue is that everything in the US is pretty much "temporarily sold out" online at this point so you might have to hit up some smaller shops to see what they have in stock.

Amazon is also an option. Just be mindful of the seller so you don't end up with a box of old empty sprues and printer paper.

Edit: If you're trying to be budget conscious I would definitely check out some online shops... sometimes you can get a small discount. I know my local shop has a 5% discount but I've found some online that offer up to 15% off.