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I don't know much about dwarves, but allying dwarves with dwarves might not be such a good idea as the troop are similar, so they have the same strengths and weaknesses
This is false. Durin's folk have no access to spears, expect the specialist Vault Wardens, while Erebor Dwarves have no access to bows. The only weakness that they share is their lack of mobility that could hamper you in some scenarios, if you do not compensate for it otherwise, for ex. allying them with cavalry.
I would definitely suggest you buying a box of Dwarf Warriors, a blister or two of Khazad Guards, and most certainly Floi Stonehand. His special rule is extemely useful to Dwarves. For 500 points Durin is not a sound choice as he is a higher level hero. In 500 points the better options you can include would be either Gimli, Balin, Floi, a Shieldbearer and obviously Captains. I am not so sure of how the Rangers perform with the nerf archery received in the new edition of the rules.
And welcome to the hobby!