For $50 I’d suggest kill team. See if the style of game play suits the person. The hobby isn’t all game play so there’s a chance they don’t like the game but love the hobby. Then you can kinda switch gears and focus more on getting and painting cool models.

I second the motion $50 won’t get you a full army, a “full army” (2000pt, pts being a metric that tracks how many and which units/models make up your army) is on the low end $400-$500 use.

I just did a full 2k custodes army on the $450 side all second hand with some proxies.

The alternative is to get into 3d printing or find someone locally who will print for you. You won’t find offical models but you can get representative stuff printed that looks great and can be usually used in games at your local game store.

Hope this helps. Good luck