I haven't actually brought anything yet and wanted to arm myself with some questions before speaking to any sales people.
Oh, my bad, I took your "I'm having to use a Cisco ASA 5506 for a VPN connection with someone and I'm unsure exactly how the service contract / software update stuff works with them." as present tense.
In that case, I'd go to your local version of https://www.cisco.com/c/en_uk/buy.html
and work from there. Look for the list of approved resellers. Ask for pricing on the unit plus 3Y or 5Y support bundled, as options. Support is normally cheaper when bundled with the hardware sale, the future price of an annual renewal is where these guys make the money from already-locked-in customers. Support pricing typically rises steeply from year 5. Ask for pricing for a 2 or 3 different tiers, e.g. 8x5xNBD, 8x5x4, or 24x7x2, to give you options (to present, if necessary) -- which ones will depend on what you're comfortable with and the risk appetite of the organization.
HTH and isn't teaching you to suck eggs!
Desktop: E3-1270 v5, X11SAT-F, 32GB, RX Vega 56, 250GB BX100, 2TB Ultrastar, Xonar DGX, XL2730Z + G2420HDB
HTPC: i5-2500K, DH67GD, 6GB, GT 1030 SC, 250GB BX100, 1.5TB Barracuda, Xonar DX