Quite a lively discussion here

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Jims, I am currently in OBX, and I'm quite glad to have a foil to play with on the lighter days. For me, foiling has not increased the days on the water, but that's because I used longboards before foiling. But as much as I enjoyed longboards, I now pick a foil with a 7.0 over a longboard with an 8.5 every time. On the foil, I'm usually foiling 1/2 of the speed I get on slalom gear (in more wind - 16 vs. 32 knots this trip, for example). But foiling is just about as much fun, even after more than 250 sessions.
To get started, look for a used Slingshot Infinity 76 (or possibly 84). It's a great foil to start on, and you should be able to find one in decent shape for $800 or less. Many foilers switch to different brands after a while, but the Slingshot foils are what most beginners in the US use, so there's a big used market. Another advantage of Slingshot foils is that it is easy to get shorter masts (61 cm and 71 cm), which you'll need for foiling in OBX most of the time.
OBX is a great place for foiling. The wind here tends to be quite constant, which makes learning easier. However, you need to watch the water levels, and know where to foil. For the past 2 weeks, the water levels have been high enough to allow foiling at Ocean Air (Island Creek) and the Canadian Hole with 72 cm masts. At Ocean Air, you may have to walk past the wind meter at low tide, but it's deep enough about 100 years beyond it for mile-long runs. If you get a few days of high pressure or easterly winds, though, the water levels may drop, and a 61 cm mast (or longer walk) may be needed.
The place in Waves where ABK Boardsports does the clinics is even better, since the water remains deep enough all the time, and often even allows the use of 90 cm masts (after a somewhat longer walk). You should definitely try to do a camp in Hatteras next may. Andy Brandt is great at teaching foiling (and winging now, too). He also has demo gear for teaching, in case you don't find used gear before then. If you can't do ABK, check with the folks at Ocean Air about private lessons.
When you start foiling, you'll need almost as much wind as you'd need to get planing, so OBX with its better winds will be a better place to learn than your local lake. Once your skills improve, you'll likely find that 10-12 knots are enough for foiling with a 6.0. One of the things I love about foiling is that it works much better in gusty winds than the slapper. There are plenty of days at our home spot in Cape Cod where the windsurfers spent most of the time re-rigging or waiting for the long lulls to pass, while a few foilers (wind, wing, and kite) all remain on the water, having fun.
As for the low end to be gained by (freeride) foiling, we've had a couple of light days here that gave a nice illustration. For a light, highly skilled windsurfer willing to use large sail, there may not be any gain. One day here when meter reading were around 10-11 mph, the only person planing was a women on what looked like an 8+ m sail and large slalom board; at the same time, both my wife on a 6 m wing and I with a 7 m foil sail were struggling to foil, getting only a few rides. Chances are the windsurfer was both quite light and highly skilled. Several other windsurfers on the water at the same time, with large sails, planed even less than we foiled.
But yesterday was probably a more typical example. Wind meter readings were about 13 knots, and of the handful of windsurfers out there, not a single one was planing consistently. Some of them got going in gusts, but it often looked like "half speed", and/or were slogging again a minute later. At the same time, my wife and I on 6.0 wing / 7.0 sail where foiling all of the time. I'm around 90 kg; someone with your weight and average skills should have had no problem foiling the entire time with a sail in the 5 m range.