FIf you use your own SMTP that's the one you want to use in your SPF record
FFor this we have to include all IP's that will using the SMTP server.
Actually you need to publish your sending infrastructure IPs e.g. your SMTP Server
FI am sure that's how SPF works 
FBtw. you also should publish DKIM and DMARC
FWhat SMTP Provider do you want to use?
FOftentimes they provide guides for this.
FGot it
Fno, reverse dns lookups should be made from the customers domain to the sending ip
F
Fwith this a receiver can verify that we actually agree that this IP can be used to send mails
this works alongside the SPF which verifies that a server can send for a domain
this works alongside the SPF which verifies that a server can send for a domain