Steve,
Most riggers will tell you that any SS, especially 304 SS, is highly suspect after 20 years on a sailboat. If you are headed off shore I would definitely replace SS chain plates, SS mast tangs, SS toggles and clevis pins, wire/swaged end fittings and any heavily loaded SS deck fittings like the deck padeye for the stay sail stay. According to one highly regarded rigger in Hawaii the above is the order of most likely failure. Warren of Sea Notes' father can make all the SS items for you of 316 SS at a good price. You should consider going one size up on all fittings as 316 SS is ten percent weaker than 304 SS.
Bronze turnbuckle bodies and Stayloc/Norseman end fittings will last a lot longer but bear close watching. We are still using our original bronze turnbuckle bodies and Stayloc end terminals, but we do a complete rig check several times a year. I have not had any issues with these items in the 16 years I have owned the boat.
You could do all the same size wire but it is not necessary and adds a little weight and windage aloft. If you had all swaged end terminals and were buying all new mechanical replacement end terminals then it might make sense. Otherwise you will be buying a lot of expensive new 3/8" mechanical end terminals unnecessarily. We carry several spares and internal parts of each size. I would, however, upgrade your staysail stay to 3/8". You will be glad you did when you get into a blow and are using that sail as your storm sail.
You can certainly do all the work yourself. Read up on the subject including assembling mechanical end terminals in any good rigging book (Brian Toss or Nigel Calder). Use an angle grinder with cutting wheel to cut your wire and smooth the ends. Measure twice and cut once. Your mast is strong enough to stand up by itself with no rigging if needed but best to do the work in stages leaving some rigging up on both sides. It is also best if you do the work with minimal wind and wave action.
The two times I have done CSY rigging I did one set of lowers and the intermediates all together and then did the uppers and other lowers together. Then I rigged the spinnaker halyard taught to the fwd end of the anchor roller tray and did the headstay (using the jib halyard to lower the roller furling headstay). Finally I rigged a spare main halyard taught to the stern and did the backstay and staysail stay lowered on their halyards.
The wire length difference between internal and external chainplates is minimal and not worth worrying about. Just make sure the originals are not too long or short, and if so, adjust accordingly. When tensioned the turnbuckle screw ends should finish in the middle of the body opening. Also ask your rigger how much the new wire you buy from him will stretch so you can allow for that.
Dave McCampbell
