@HarrowMy 5 cents from the point of having started about 4 weeks ago.
What to start with?If you can get a demo / rental / loaner from someone for the first two or three sessions that would be ideal. I was lucky enough to get my hands on a demo Slingshot Wizard 125 in combo with a Ridengine foil which had a 61 cm mast.
I am 110 kgs and did my first session was in about 15 knots of wind with a 5.3 on choppy water. It was a handful. But progression is fast and after about two or three sessions you could be able to get relatively long flights with decent control of the foil.
I think if you know how to surf and and are a reasonably advanced windsurfer you should be able to attempt your first jibes after only a few hours on the water.
Helpful tips?As others have said, you should have a smaller sail than you would use windsurfing in the same conditions. Ideally you want to be underpowered... if you can easily water start then your sail is probably already too big.
Ask other people for tips and have a look at various foil instructional videos on YouTube by Balz M?ller, Foil Academy and co. Put those tips into practice as soon as you can get back out on the water even if that means you might miss a windsurf day.
What gear next?As others have stated the Slingshot i76 + 90 mast package is a great free ride foil. It's a bit more versatile than the systems by other brands and not as difficult to ride (or expensive) as the pure race foils. At the end it will probably come down to what you can actually buy. Slingshot seems readily available. Some of the other brands are a bit more like finding a unicorn.
As for a board, I think you could use a door from Bunnings and mount a foil to it and it would sort of work. The most important part is the fin box to foot strap to mast track position. On "normal" windsurf boards that setup won't work particularly well for foiling. I think that's the reason why most brands are now coming out with dedicated free ride foil boards.
The Slingshot boards are great and should keep their value given they are quite popular. If you can find a used formula board you might be able to reset the straps and put in a re-inforced deep Tuttle foil box.
Do people move over to foiling and give up windsurfing?Don't know about other people but I like wave sailing too much to give it up. But windfoiling will extend your time on the water on those lighter days in the 10-12 knt range.
Yes you could get a 10 m slalom sail for those days but who wants to fight a 15 kg rig. The feeling of windfoiling is probably more like surfing than full power windsurfing.
It's a bit like the first time you get your windsurf board on the plane. Once you get that feeling of the first flight you'll be hooked!