I am coming to a point in my game where I really want someone to look at what I have and tell me if I have decent core gameplay. It's missing polish in terms of art, that I want to get contractors for, but I don't want to go ahead and do that until I feel like I am working with a solid foundation. Hell, i'd like to know if this works on other people's computers.

I know I can make a restricted release for a game, but should I go about getting playtesters?

Well, what I do, is I just give trusted people or people Im working with a copy of the sourcecode, give them details at what you want them to look at, try all the different versions (Android, Mac, IOS, PC, etc...) And ask them for feedback, then after taking serious note of whats good/bad you polish the game, fix it up, that stuff.. and then you ask people if they wanna beta test your game.