Do not buy a Warframe. Every single warframe and weapon can be gotten for free, many of them are really easy to get, too. So buying them is just a waste of money. Especially considering the plat/cash exchange ratio. One frame costs like $15. There are FAR better things to spend 250+ platinum on, I promise. And you’re gonna realize that when you run out of weapon slots here soon
But yeah, as far as tanks go – Valkyr, Inaros, Rhino, Chroma (though he’s kinda annoying to farm and NOT newbie friendly), Nidus, and Wukong.
Valk, Rhino, and Wukong are all easy to get with Valkyr available from Jupiter, Rhino available from like Venus, and Wukong being a Dojo-built frame.
All that said, Excalibur is one of the best all-around Warframes in the game, objectively speaking (though I don’t enjoy using him myself). Between his long-range Crowd Control, respectable armor levels, and Exalted Blade, he’s got a lot of capability to be a beast.
But like Sidewinder pointed out, it’s gonna be a hot minute before you can craft another frame. Warframe is a free-to-play, so grind is unavoidable if you want to play it that way.