Hi Zac,
Hope all is well
THe things your looking at are good, id be tempted to add one more - price. There are a lot of hotel aggregator sites all using a range of suppliers, some have good contracts and low margins which gives a very competative price to the consumer, some have good contracts and high margins which is jsut greedy, and others have crap contracts and will always be expensive
Check some comparison sites ot make sure you pick someone who has good prices in the areas you wish to promote or your conversion will struggle.
Commission - watch out what figure you look at. If Agoda.com offer 60% commission whats this of? As i cant believe that its of the total booking price, a hotel reservation for £300 - are they going to give you £180 and then somehow from the remainder pay the hotel and make a profit themselves?
Most companies making large commission claims offer it on profit and you need to know what this is beofre you commit to working with them.
HotelsCombined.com is a white label comparison site, good but depends how far you want to take it. In reality your using another affiliate site to drive your sales and your relationship is with them.
Depends on how your going to market them but i wouldnt say number of hotels is a massive issue, availability is what you need to check.