Is there any limitation on line count for placing orders from SCA? 100+ lines?

Hard limit? No. Soft limit? Yes. The key constraint here is performance. Most sites experience what you might call 'unacceptable slowness' anywhere between 50-100 lines. It can be mitigated but anything over 100 is unlikely to be acceptable to users, or may just time out.

I'd be interested to hear what can be done to mitigate issues in this area, as we've just paid for an SCA license to evaluate the system and this may be a concern for us.

Well it would be pretty forensic. You would need to set up a test of your current site with the expected number of line items (eg 100) to get a baseline performance status. Once you have that, you would need to begin to trim away customisations and potentially unneeded data from it. The principle causes for concern are the field sets used by the sales order and cart

Thank you!