I would like to answer in following parts.
A.
1. SRX 1500 is very stable in terms of features (Routing + Security)
2. We have been running it for one of datacenter environment and found perfectly fine without any issues.
3. Though it does not have bgp as routing protocol.
4. However, from my experience i do not see a problem in packet mode or flow mode for this box.
B.
Your requirements are more of SP segment than for Pricing you may get in touch in SE for MX series devices.
As per my understanding MX80 may be a suitable choice.
HTH.
Accept this as solution if it resolved your issue.
Kudos would be appreciated too.