For microwave modules, aluminium is a common material used for chassis design. Plating is necessary when you want to reflow your substrate directly onto the chassis body, or you want to reflow your feedthrough capacitor. Gold is commonly used.
There are different grade of aluminium. The 6000 series is commonly used. If you need laser sealing, special care must be taken on the al material. Usually, people use 4000 series for the cover, and 6000 series on the chassis body.
Question of whether sealing is required? Depends on your module design. Do you use bare die/chip, with bond wires, or you use sealed components? Do your module operates in enviroment where water condensation is possible. If yes, most likely you need some kind of sealing.
Method to transfer signal out/into the module? You can use SMA connector up to 20 GHz with no problem. There are different type of SMA connector. Some can be used for laser hermetic sealing, some are for normal application. Check out the website of MA/COM or SouthWest Connectors for more info (Do a google search)
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