Here are my two cents:
Pretty much exactly 5 years ago I was in a similar situation to you. (but 80kg

)
Here is the order I bought things in:
1) I bought a used 155L Tabou Guru (same shape as Rocket, different construction) with 50cm fin.
Sailed it with a 6.0 Gaastra Pilot.
Big fin means early planning even with smallish sail. Makes people on shortboards jealous.
Smaller sail means easier uphauling and better for learning everything.
Lots of volume means you can have fun and learn in non-planning conditions.
2) Bought a 7.6m sail (15 + knots) (RRD Fire Mk 3?)
This will depend on the conditions of your local spot and how confident you feel going out in stronger wind. I just wanted to get on the water more often in less chop.
------------------------------- WATERSTARTING BEGINS -----------------------------------
3) Shortboard. Kept using the 6.0 Gaastra.
- This was an F2 Ride 277 I picked up for ?5 / A$9.5. It was great in the chop but the oldschool shapes aren't good for learning.
- Got a free Bic Tribal 84L which obviously I was hopeless with due to the volume and my skill level.
- Finally bought a 113L Starboard Kode which I still have today. The modern shape is great.
4) Smaller sails.
As confidence grew and waterstarting improved, I was able to go out in stronger wind.
5) Got a 9.8m sail (Spacedog Speedster) to use with the Tabou Guru - worked great in 10-15 knots.
Wanted to go out in very light wind during the UK summer.
6) Sold the Tabou in search of more speeeeeeed and got a 145L slalom board (Exocet Warp 80).
7) Bought and sold lots of cheap boards as I like trying different ones. (JP FSW 84, JP FSW 98, Naish Hybrid Slalom...)
Now, I have become sensible:
Boards: 94L Kode, 113L Kode, 125L F2 XS
Sails: 4.2m, 4.7, 5.7, 6.7, 8.6m (The spacing still isn't perfect)
Will potentially become more minimalist and replace the 113L and 125L with a foil and sell the bigger sails.
Basically, get a lightwind big board and you'll be able to keep it forever unless you're like most of us who end up buying and (occasionally) selling kit

Oh and 1 good sail to start off with.