As I think I stated in the article about fins, fin size can depend on several factors.
When you change boards, sometimes the difference in width, as well as the design intent (wave, freestyle, slalom, freeride, race, formula) can make your existing fins much better for the board you are selling, and much less satisfactory for the board you are changing too.
Your AHD Inspiro has the following specifications (If I found the right info on the older AHD website):
volume : 136 l.
length : 252.00 cm/8' 3 ''
width : 78.20 cm/30 3/4''
width ofo : 47.40 cm/18 3/4''
weigth : 7.80 kg.
finbox : Tuttle Box
fin size : 40 - 54 cm
sail size : 5.5 - 10.0 m2
OK, so here are the specifications for the 2008 Futura 111:
Length: 244 cm/96.06"
Width (ofo): 43.7/17.20"
Weight: 7.0 Lg (WOOD) 7.3 Kg. (Technora)
Fin box: Tuttle
Stock Fin: Freeslalom Swift 380
Sail Size: 5.0-8.0 30-42
So, if we compare these 2 boards, it's readily appearant that the Futura 111 is approx. 10 cm/ 3.937" narrower overall, and 3.7 cm/1.456 narrowewr at the one foot off the tail (ofo) measurement.
So, using your existing 50 cm fin might work, but your board would be somewhat prone to tailwalking and other symptoms of having too much fin.
I will now address your specific questions.
My questions are:
1. I have a 50 cms North Shore fin that I was using in the old board with the Gaastra 8.0, is it appropriate to use with this new board with less volume ?
The volume is not the problem here, it's the overall width and the ofo tail width.
Since the tail is narrower, you won't have as much "footstrap offset" (the distance out from the fore/aft centerline that the footstraps are mounted) so controlling a fin as large as 50 cm would probably be OK as long as you are underpowered and not at full speed, but as soon as you get the Futura 111 up to full speed, or try to get the best vmg (velocity made good) upwind, you could see some control issues.
2. The 38 fin delivered with the new board, is it for what range of sails ?
Normally, the stock fins supplied with Starboards are kinda for the "mid range" of both sail size and sailor size. You are not real heavy, so I'd think the stock 38 cm would be best in the 6.0 m2-7.5 m2 range for a sailor your size.
3. What sizes of fins, do you think I need in my quiver with this new board ?
If 38 cm is the stock size, I'd think a 42 cm and perhaps a 32-34 cm would give you the
4. Finally, what kind and brand do you recommend (Tectonics ?) ?
Tectonics is good, but there are other brands in Europe that are equally good I'm sure.
Select fins are good mas produced fins, or you could get something from Deboichet in a custom slalom fin also I think.
Also consider the Lessacher Design fins from Germany.
Wolfgang has some very fast designs.
Hope this helps,