Lock a server within a rack of several servers

Buy them. And buy them from alternative sources if you need to be frugal - Craigslist, Ebay, Dell Outlet, etc.

If you end up building them - go with SuperMicro - great gear.

But Commercial Servers will have better out of bandwidth management, better systems management, better support, etc.

And if you need to pinch pennies - use third party memory (i.e. Crucial) - its cheaper and just as good.

If your servers use front to back flow-through cooling, as most rack mounted servers do, leaving gaps can actually hurt cooling. You don't want the cold air to have any way to get to the hot aisle except through the server itself. If you need to leave gaps (for power concerns, floor weight issues, etc) you should use blanking panels so air can't pass between the servers.