Need help deciding what unit to buy next

I want to use my Christmas money to expand my army, but I'm having trouble deciding what to buy. I know I want to buy something heavy because fighting tanks has been a recurring problem for me. I already have a Land Raider, Predator, Razorback, Blackstar, and two box Dreadnaughts, but they either don't do much or go down quickly. Also, a lot of the people I play with like running armies of mostly tanks and heavies while I have a mostly infantry army. So I'd like to at least have a chance against them and not get all my best stuff blown off the board turn 1.

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