Off the Shelf? Fairlight Caliban with Novatech Navi OS. That will get you a 'link with Response 4, Signal 5, Firewall 3 and System 4. With access to the
War! books, the only one you can buy under Availability 12 is the Singularity Battle Buddy Basic 'link with Resp 5/Signal 5 for Avail 8R and 10,000¥.
You may want to check with your GM and see if you can mod the 'link at character creation (if not, then work out what you need to do with him to mod it in-game). Upgrading the Fairlight/Novatech to it's max (Core rules max, that is) brings it up to Response 6 (8,000¥), Signal 6 (3,000¥), Firewall 6 (3,000¥) and System 6 (3,000¥). You could code the Firewall/System yourself using an extended Software Test (Threshold 12, 3 months for Firewall, 6 months for System) and build the Response/Signal with an extended Hardware Test (Threshold 12, 1 day for each). Of course, these levels are all restricted gear for new characters, so you'd have to pick up the
Restricted Gear quality from
Runner's Companion to get them. Oh, and you can only get that quality 3 times, and there's four pieces to the 'link...

Of course, if the GM allows it, you
could get MilSpec 'links and software. But that is with EXPRESS GM APPROVAL. The 'links with Resp/Sig above 6
start at 24F Availability and max out at 48F for the Transys Cybernaut (R9/Sig8) (not even the
Restricted Gear quality will all them). And Firewall/System ratings from 7-10 have (Rating x 4)F availability and (Rating x Rating x 500¥) Cost. The utmost max you can go is Rating 10.