What specs should I be focusing on that will ensure the stability and reliability of the
None. Seriously, a SOHO router that is not reliable and stable is a defective product and should be replaced under warranty. I am always surprised by the amount of bullshit people take these days for low end routers. That thing is not stable is not acceptable. Been using SOHO for years before moving to Mikrotik (I used AVM) and never had any problems.
Company is expanding from a 30 person office to a 100 person office
That puts you maybe 900 people below enterprise size ;)
Seriously, get a decent mid range router. Either if you know how to use them from Cisco, or if you want something that works as good and is cheaper - a lot - from Mikrotik.
The exact model seriously depends on what you do - how much internet you have etc. - a 100 people architecture company may not need as much bandwidth than a trading company. Without more information given that can not be answered. It also depends what you plan to do with this bandwidth.