Either a 30 or 40 cuft tanks would be enough to get to the surface from a depth of 100ft in a dive that doesn`t exceed the NDL of your tables, provided you don`t panic and and your RMV stays below 10 cuft/min/ATA (a ridiculous number). (Basically, yes, yes they both will surfice.) And if you ever go into deco or trimix diving, these are the tank sizes you`ll need for stages. As far as brand it really doesn`t matter for now an aluminum tank would probably be easiest for you to care for; all of my stages are 40 cuft Luxfers.
Now the next few answers refer to having the bottle slung as opposed to mounted on the side of your primary, like you want, so choose to ignore these suggestions if you`d like. But I still think they`re something to consider.
For a regulator go with a DIN first stage as they are much more reliable, and even when I`m using my bottle as a pony and not a stage it`s still slung, so I charge the hose pre-dive and turn it on if/when I need it, DIN keeps the first stage from separating from the valve should I lean on something and purge the system. This method prevents free flow, and I probably should have mentioned get a DIN valve on the tank, and if you choose to go yoke, you can buy an insert that converts the DIN valve to Yoke. (Again, if you decide to do Deco/Trimix dives you need a DIN valve. Plus if you decide to sell the tank it`s worth more.)
For an SPG check leisure pro or scuba.com for a cheap one, but I have a six inch Dive Rite SPG`s, but again mine are slung not rear mounted so I don`t need a long hose.
I don`t have advice on mounting it to the side of the tank, other than that there are a variety of mounts, clips, gadgets, and accessories that achieve this function available commercially. Just remember to put a counter weight on you left side to counter the additional negative buoyancy on the one side.
Enjoy.