What I think,
make the board with the footholds to climb the mast, only make the board thicker under where your feet go, as all your weight is pressing on a 12mm wide (or so) timber - wider is easier on the feet, unless you have strong soles to the boots. You can add a bosun's chair and a person to help, but the foot board will give your feet something to hold on to up there.
On the rigging, yes, replace the running rigging first, as that will haul you up there and hold you there, and be the back-up safety lines, etc.
The main way to save money depends on the rigging size. For 5mm wire or less, you can bend it around an eye if it is 7x7 or 7x19 wire. 1x19 is too stiff for bending around an eye. After that you can put in turnbuckles.
For my lower stays I did it in 5mm galvanised wire, at about $3 per metre, plus a dollar or 3 for the eyes and the same for the swages. The turnbuckles (stainless M8 or M10) cost me about $30 each. You could use stainless 7x7 wire if you prefer, up to $5 per metre at these sizes.
A friend put me on to galvanised wire, saying how cheap it was and that it was commercially commonplace for cranes and lifts/hoists, so will suit yachts. It did so in the older days, before shininess was paramount. But it still works today. Maybe you replace it after 7 years instead of the ten or so for stainless rigging.
I've swaged 6mm wire and it was far, far harder than anything up to 5mm. I use a hand swage tool, Bunnings sell the ones that do 1,2,3,4 and 5mm for about $90. You don't need a hydraulic press or to hire one or to have a shop do it.
OTOH Miami Stainless sell 6mm swageless terminals for $34
www.miamistainless.com.au/general-hardware/swage-terminals/swageless-terminals/12x120mm-s7810r-0612120 as well as wire
www.miamistainless.com.au/wire-rope/per-metre and every other fitting.
Last year my 5 outer stays for a 28' boat, cost about $1550 for the shop to do the lot, including replacing 5 sheaves and pins.
So it all depends on the wire size and the terminals you have on the mast if you want to DIY with eyes and swages, or DIY swageless fittings, or just budget the figures quoted if you delegate it all to a shop.