If the slope is relatively shallow (less than 4" per foot) use a peel and stick underlayment under the shingles in lieu of standard felt (or often referred to as tar paper). Cover the whole deck with the peel and stick material prior to installing the shingles. Also, if you aren't going to re-roof the existing roof, remove enough shingles on the existing roof to allow the peel and stick to extend minimum 24" beyond the valley centerline. The peel and stick gives you and extra layer of waterproofing protection under the shingles and helps with what will be a relatively shallow valley. The material also has some sealing capability around nails.
If the sunroom is not conditioned space, you do not need to ventilate the roof. However, if the room is conditioned, you should try to install some type of ventilation and that is not easy with this type of add-on if you have a vaulted interior ceiing unless the space is open into the attic of the existing house so that you can install soffit vents that allow air intake flow through to whatever type of roof ventilation you currently have . If the space will not be open into the existing attic and you need to ventilate a valuted ceiling - good luck. If you can frame to create some cross ventilation space between the roof deck rafters and the ceiling joists you could install vents in the soffit and end wall(s) to allow for the recommended ventilation.
I hope this information helps.