Replacing a Premier Keyset System - Questions about SLA, etc

I am attempting to replace a 25 year old “Premier” keyset system at my father’s business with an asterisk based system. The current keyset has some interesting features that I am wondering if I can replicate in Asterisk.

I will be using 4 analog PSTN lines with the possibility of 1-2 SIP based lines from broadvoice.

Single Line Appearences: The current system works like this, place a call on hold, pick it up at any other station. I know that asterisk can work like this.

The iffy part, from what i’ve read:
When you pick up a keyset on the current system it drops you into a local dialplan where you can dial another extension and have it go into intercom. You either press “9” or select a line button to grab an outside line.

From what I have seen in the SLA documentation, It does not seem that I can pull a phone off hook and drop it into a default context, it has to grab a trunk.

Am I wrong? What way should I approach this problem? Also, I am planning to use GXP-2000s, do these work well with intercoms?

Thanks, Please ask questions if I am unclear!