I've always done them horizontally, because that way you always have clear access to the tang with a screwdriver. If you install them vertically there is potential for good access to the tang to be obscured by equipment above or below, or the top or bottom of the rack.
It's really difficult to buy an unsuitable rack today if you're purchasing new.
I usually encounter generic, custom or unbranded racks, APC Netshelter, Dell and HP (10642 G1 and G2 models) in the field. They've all been solid and have handled the systems and equipment I've needed to mount within them.
The basics:
Begin with your servers. Why not match up with the equipment you're currently using? If you're on Dell servers, you know Dell racks will work. Same for HP. That's a very good starting point.
In addition to manufacturer compatibility, you'll want a four-post, square-hole rack. At this juncture, you should not consider anything else for housing servers. That means NO round-hole threaded or unthreaded round-hole racks! Other answers here elaborate on the reasons why, but again, it's difficult to end up with a round-hole rack if purchasing something new today.
Do you want sides and doors? Sides are optional in situations where you make want to bay/bind two similar racks together side-by-side. Also, if you need easy access to systems and cabling, you may want to omit doors/side panels. Is there a security concern or a need to restrict physical access to the systems? If so, the doors and side panels are worth getting.
Networking gear could mean switches, or switches and patch panels... While both of those work best in 2-post telco or relay racks. They can live happily in a 4-post rack, but would ideally be mounted in the rear rails versus the front of the rack.
Think about were your infrastructure cabling will terminate. E.g. do you want to drape fixed wiring inside the cabinet? You want to absolutely avoid this scenario.
You'll want shelves for any non-rackmount equipment (tape drives, Cisco 1700 routers, KVM switches, etc). Try to buy shelves purpose-built for the chosen rack. (e.g. HP shelves with HP racks... APC with APC...)
I don't usually bother with in-rack cooling. Proper rackmount servers and equipment pull cool air in from the front and exhaust hot air out the rear. A top-mount fan really hasn't been particularly useful in my environments.
I skip the dedicated KVM/monitor/keyboard in racks these days, opting for a more portable "crash cart" or a laptop-KVM adapter.
Ensure depth is appropriate for your preferred power distribution choice. APC racks accommodate their PDU's quite well.
Full rack enclosures are often delivered whole, palletized with casters. There isn't any assembly really required, other than removing them from the pallet anchors. Open-frame racks may be shipped in one or more flat boxes and require assembly.
Do you want a permanent installation, or do you want casters in the bottom to allow the rack to be moved and repositioned easily?
Outside of those items, you'll be well-served with most recent APC (universally-accepted, very adjustable), HP or even Dell racks. Quite a few data centers use APC racks, or at least use similar designs since they're in the business of accommodating a wide variety of customer equipment. You can also opt for an open-frame 4-post rack if this is a permanent installation.
Rack Solutions is also a great (re)source and can answer any specific questions or give you some design ideas.
Examples:
Custom rack: Permanent installation. No casters, front view with shelves, side panels and door attached.
HP 10642 G1 rack: Movable rack on casters with side panels installed, door removed, fully loaded with big servers, UPS, CRT monitor (LOLWUT?), KVM, 1U pull-out keyboard/mouse and two shelves.
Dell rack: Rear view. Split rear door for clearance and ease of access (versus a one-hinge full door). Side-mounted cable management loops.
Custom rack: Rear of a custom data center rack enclosure with patch panels and networking facing the rear. Proper clearance for PDU's and server rails.
Open-frame 4-post rack No doors, side panels or casters. A skeleton rack, essentially. Good for airflow or easy access to cabling.
Best Answer
Supermicro offers a Threaded Rail Adapter Set for Round Hole Racks (MCP-290-00060-0N) which sounds like what you're looking for exactly.