Upgrading a ''''93 Hunter 27
While none of these changes are imposible some are much harder than others and IMHO some just make no sense at all on a boat this size.
Tiller to wheel steering:
Probably the closest to imposible is to change tiller to wheel steering. To properly install wheel steering on a 27 footer you should have a ''tee'' cockpit so that you can have a big enough wheel and room to get around it. This means building a new fiberglass cockpit with the proper configuration. When you are all done you will take a pretty nice sailing boat and damaged its sailing ability by adding the additional weight far in the stern and also reducing helm feel. In my book, wheel steeing does not really make much sense on modern boats under about 32 or so feet and certainly would be a major negative on a comparatively light weight 27 footer. Frankly this whole wheel steering on small boats seems more like an affectation that a soundly based idea.
-change portapottie to marine head with holding tank:
That is comparatively easy to do during a haul out. Minimally you need a head (of course), holding tank, deck pump out fitting, a thru-hull, a seacock for the water intake and all of the necessary hoses to hook all of that up. If you sail on the ocean and go 3 miles offshore, or are outside the US or just want to flaunt the law, then you need a discharge through hull and seacock as well as a Y valve.
Without the discharge you are talking $400 to $700 with reasonable quality equipment. Add $150-$200 or so if you do the overboard discharge.
-add roller furling:
Its easy to do, call Harken and just add money. This one also strikes me as an unnecessary idea if you are in reasonable physical condition. Folding, flaking and stowing a jib on this size boat is a peice of cake. (I have had a boat this size for the past 13 years.) If you have a hank on jib leave the sail hanked on with the cloth between each hank pulled through to the oposite side from the hank below. Then starting from the clew, pull out each fold working up the leech pulling against the hanks. Once flaked simply roll the sail into itself. If you can get ashore to flake the sail (and this works well on large genoas) Tie the head to anything and then standing at the Tack of the sail simply fold the sail into full length triangles parellel to the luff and maybe 18" on the bottom. Then walk to the head and flop fold it into the sail. If the sail is on a headsail foil (as mine are) it gets harder but not imposible. I simply stand on the leeward side of the boat with the sail on the windward deck and starting from the head, roll my jibs.
-add shore power adequate for portable AC
This is not too hard to do. Contact the American Boat and Yacht Council in Edgewater Maryland and get a copy of the standards for wiring a marine 110 system for a boat. The Anco wire webpage used to have a copy of the standards as well. In a rough sense you will need a 30 amp shore power cable(big enough for a small HVAC unit), a 30amp shore cable connection. A 30 amp main disconnect/breaker within 10 feet of the panel (closer is better) a panel with circuit breakers for each circuit, a marine ground fault protected receptacle, a proper ground, proper marine grade tinned wire and proper connectors. The wire needs to be properly supported and the terminals need to be a proper crimp type for 110 circuits. Wire nuts are not legal. All connections must be shielded from accidental contact and be labled as 110V on the outside of all access panels.
-add H&C pressurized water:
That is not too hard. You will need a pressure pump. For a boat that size I like Shureflo pumps. They are cheap, reliable, have internal pressure switches, and the rebuild kits are complete and easy. They are a little noisy and you will need the largest cpacity they make if you have both hot and cold water. You then will need find a place to install the hotwater heater. That will not be easy on a boat that size. Assuming you have room you need to buy a hot water heater, hot and cold fawcets, and some hose, a 110V 15 amp breaker for the panel you installed for the HVAC unit and some 110 volt wiring. Most small marine hot water heaters can are heated off of the fresh water cooling leg of the engine and by 110volt current at the dock. You will need to select a less than 15 amp water heater and less than 15 amp Acv unit is you plan to run both at once and size your wiring accordingly.
I don''t know that there really are kits (except for the roller furler) for the extensive changes that you are proposing. Parts for this kind of work can be ought off the shelf with a little research and care.
If you do all of this you will have put a lot of stuff onboard and the cummulative effect is to reduce performance and add a lot of complexity an maintenance to your boat. To some like me, the head is a god idea but if you did everything on this list you would have a lot of money into this boat and people like me would see the boat as being diminished in value by quite a lot. On the other hand, some people would see the boat as enhansed but certainly not by the total cost of this extensive list of changes.