To be honest, out of all those bits the only parts I'd consider usable for a decent Router at this size is the Long length of profile and rails. The shorter profile with belt attached is not really strong enough for a Gantry, it would make a great plasma gantry just as it is but not really very good for a router at this size.

A belt drive system is Ok and in some ways better than R&P because it's much more efficient with less backlash but as you know they do start getting a bit flappy when you start going past 2mtr's so it's not something I'd use at this size unless was looking to do it on the cheap reusing what you have. However, the work involved in building such a large machine is high so I'd always try to talk someone out of doing it on the cheap because it never works out that way and often leads to disappointment or failure.
In fact, I'd try to talk anyone who is new to building CNC machines out of building such a large machine for their first project, it's common for people to think that it can't be any harder than building a small CNC as it's just scaled up but believe me it's very different and much harder if you want anything worthy of the BIG effort required.

My advice is to make a plan don't try to wing it and build/design as you go along! It will take you much longer and cost you more money as you'll scrap half of it in cock-ups or design changes. Taking a few days or even weeks to sit down and make a plan/design will pay back BIG time and you'll be cutting sooner with less frustration.

Here's the best and proven way to successfully build a great machine.

Do the research, make a plan, bit more research, change the plan, start a build thread, ask questions, do bit more research, few more Questions, finalize the plan, Start the build, THEN AND ONLY THEN BUY ELECTRICS

Good luck