m18 or older models?

Dell availability of older systems is subject to what you see on the website (or refurbished site).

Other vendors may have older models in stock -- basically you'll need to search for what you want.

Can you share the URL? Thanks

I only found the alienware x16 r2, but it's out of stock.

Try newegg.com, amazon.com, Best Buy, etc.

For refurbished -- https://www.dellrefurbished.com/

https://outlet.euro.dell.com/GDOOnline/Online/InventorySearch?c=uk&l=en&term=Alienware%20m18

I found this, but I'm afraid if it's used what condition it will be in. For this money I can get a new one and not have to think.

The systems in the Outlet will always be cheaper than a comparable new system so you just need to check what specs each one has. Some Outlet systems will seem more expensive than some new laptops but the specs will not be the same. 

For instance, the refurbished m18 R2 in the link above is almost maxed out for 2,300 but the lowest spec brand new 18 Area-51 is listed for 2,300. Same price but different specs and you can get a new Aurora 16" for much less than those. All still come with 12 months Dell warranty. 

I would suggest to go for the newer models with the newer generation parts and newer features. 

I've ordered laptops and monitors from the Outlet before and never had a problem. They have always looked brand new but you never know what you'll get. I would only go for refurbished though if they are significantly cheaper than new. For a few hundred bucks it isn't worth it. 

Thank you very much for the advice. I think I'll do that.

one more question

Has anyone managed to install an 8TB SSD?

I'm thinking of the new Alienware Area 51

OWC 8TB M.2 NVMe