I just did a "budget friendly" panel upgrade so I think I can speak to this topic:
1. Autopilot: Focus on the autopilot as the backbone of your upgrade and strongly consider the GFC500. This will decrease your workload significantly and decrease fatigue. In my opinion, mandatory for single pilot IFR and extremely helpful for VFR.
2. GPS: If you are only flying VFR, do not worry about additional GPS capability, but if you are planning on flying LPV approaches then you need WAAS. ADSB is another consideration for US airspace and you would need WAAS with a paired transponder to satisfy this. Do not shy away from an affordable GNS430W, it will get the job done can be replaced later if needed, the new navigators are costly and would put you over budget to be able to afford an autopilot. I kept my 530W and its totally adequate.
3. EFIS: I believe if you are interfacing with older navcomms / etc you are best with the 275, but if you have a 430w it should work with dual G5s. This will also allow vac pump delete. I chose to upgrade my #2 navcomm to the GNC215 to allow it to talk to the dual G5s, allowing me to remove all other CDI / glideslope instrumentation.
4. Flight director: This is a function of the autopilot (GFC500) and not the EFIS. The flight director presents hollow magenta bars for which you to follow (as if you were the autopilot) to facilitate more precise hand flying. When the autopilot is active, the magenta bars become solid and the AP servos engage to follow the flight director cues. To my knowledge, a 275 on its own will not give you a flight director. My G5s present flight director bars only when the FD is engaged on the GFC500.
5. Timing/finances: do not let an avionics upgrade compromise your ability to take of other incidental mechanical maintenance issues that may pop up.
6. Depending on your location, consider a Canadian avionics shop, take advantage of competitive exchange and labo(u)r rates eh!
7. Don't forget good engine instrumentation, much more important than an avionics upgrade in my opinion. A JPI730 will do the trick.
FYI my panel now consists of Dual G5s, JPI EDM 730 (existing), GMA340 (existing) GNS530W (existing), GTX335 (existing), GNC215, GFC500 (3 servos). This came to ~35k USD installed which I think was reasonable value. For a 20k budget perhaps you could get a new audio panel (sounds like you need it, maybe find a used one) + GFC500 with a single G5, and keep everything else for now. If money to spare do the 430W upgrade and transponder.
Best of luck!