Well the build logs are all machines that people have built themselves (well most of them) I would take my time rather than rush out & buy something that might not be suitable, don't know anything about the Chinese machines so no idea what components they use so not sure how much of the frame would be worth using. For supported rails, ballscrews & nuts + the support blocks/bearings a lot of people on here have said that Chai is good to deal with he is on the auction site as linearmotionbearings2008 haven't used him myself but would on the recommendation of some of the members on here. Aluminium can be bought from aluminium warehouse (just google it) & for profile rail can be bought from KJN, again not used them myself but they have been mentioned a couple of times.
First thing to do would be to sort out a design which you could do yourself from scratch or take bits from machines you like in the build logs. People will help you get the design right for what you want the machine to do, then you can start building it but just be aware that designs often change a few times before the machine is finished lol.