It can be fun but it can also be frustrating as well as saddening depending how things go so prepare for disappontments as well as fun. I'm just being realistic after being in the hobby for a while.
Personally I would go with a brand that has everything you need as far as tank, stand, and plumbing in one box. You supply pumps heaters etc.
Custom is awesome but until your in the hobby for a while and figure out exactly what you want as far as livestock I'm not sure how you would even begin designing a custom system.
I only have experience with one brand you mention, Redsea. I've been happy with mine so far. My reefer 170 is the first image and has been running for over 4 years. My reefer XL 300 g2 has only been running since February this year. It is the second image.
Reefer 170
Reefer 300