As others have already pointed out, a longer SMB (6') would be better for locating you (find-abilty).
Baffled bottom with OPV and oral inflator. I generally use either my octo or a 36" dedicated bcd inflator hose to fill the SMB at depth. It fills FAST when using the inflator option and one needs to be prepared for it, but it is a breeze to do. The baffled bottom allows for the SMB to remain inflated at the surface if laid flat (well, they will leak a little). The baffled bottom ones I use have magnets to keep closed (as well as the duck bill valve). I find if using these from the bottom it easier to use your fist to open then fill. Color is another subject. I have both a green and orange one.
As far as a reel, that depends on what depth you are sending it from. A spool is fine for sends at your safety stop.
If you like or need to send from depth, a reel shines here. Although, a spool can be used of course and I have done this many times. Kinda like snorkeling after learning how to dive, meh, I will stick to diving. I will keep using a reel. If you do shore or drift diving as well and need to tote a flag, having a reel fits both purposes.
What type of reel? Lots of options (and opinions) available in this category.
In OW, I prefer a ratchet/thumb reel. They are easy to deploy and manage with one hand (if you need to release line, it can be done easily with hand you are holding the reel with using your thumb). Most ratchet reels are hard to mess up being they wind only in one direction when the mechanism is used. There are other reels with different types locking or tension mechanisms and I sure someone will chime in with their preference.
No matter what reel you choose, you probably will still need a spool. Where I dive, if I am 'elected' as a dive flag tender, I use my ratchet reel, but keep the SMB and spool as a kit at my side JIC I loose the reel or whatever.
