I am going to play a little devil's advocate here. This *could* be the plane for you, if you get a price you feel is fair and it sails through a reputable pre-buy. OTOH, it could be an awful option. IMO, there just isn't enough information to make a recommendation. What is the mission? You mention you'll be flying your wife and kid around, but how often? Will you be using it solo, or will it be primarily a family hauler? What will be the typical trip distance? Are you robust or slight people? Do you travel heavy or light? Will there be any time pressures anticipated with your usual trips? What are the terrain and DA considerations around your region? Is this a forever plane, or a stepping stone to something more capable in the medium-term future?
To directly answer your questions:
-Interior: as stated by multiple people above, plan for a few AMUs if you DIY it, or 10-12 AMUs if you go for a completely hands-off approach. Remember though, this is really just cosmetic and could be done piecemeal and as funds allow.
-ADS-B out: $2500 is about right for the most basic options, like uAvionix.
-Autopilot: You'd be kidding yourself to think you could get it done for anything less than 12-15 AMUs installed price for a basic 2-axis solution like the Trio or an Aerocruze 100 (if they are available for Cardinals). You will get quotes of $20-25K. There are reputable shops out there that will do it for $15K +/-, but you might have to wait a bit longer to schedule, and/or fly it halfway across the country.
On the note of avionics. This is just my $0.02, but with that panel, it would be foolish to sink the kind of coin an AP would cost without adding a modern GPS navigator. Given that you need ADS-B out, your most financially efficient route would be to install a GNX-375. This would give you a modern IFR GPS navigator that can integrate with your AP, as well as include a built-in ADS-B out transponder, for around $9K for the unit plus a couple AMU more for antenna and misc accessories. Depending on your budget, this would also be a good time to pull the vacuum system and replace the AI and DG with G5s (or GI-275s for a little more), figure 6K in parts for G5s plus a few thousand install. You'd recoup some of that selling the working vacuum pump and gauges, plus you'd save the cost of a CDI for the GPS. You could probably get a GNX-375 + basic 2-axis AP + dual G5s installed for 35 AMUs all-in. It would add 15-20 AMUs to the value of the plane, so it wouldn't be a total money burn. If you were on a stricter budget, subtract the G5s and you're probably looking around $25K.
It sounds like you want a plane that has a decent panel and a pretty interior (don't we all). If you can get a good price for the plane, then maybe $35K into the panel and $5K + your own time/sweat/blood into the interior is worth it. First and foremost though, the plane needs to fit the mission and have good bones. If not, don't settle for something that won't work. All the pretty glass and diamond-stitched leather in the world won't make a plane go faster, climb quicker, take off or land shorter, deice better, or carry more.
