8 oe 9 knots will get you there and back...you'll have some diesel to spare for rushing after convoys and whatnot, but not that much...
The KDB is a must, it solves the problem of the port and starboard 10 degrees off your bow not having good hydrophone coverage...
The diesel upgrade will slightly improve your top speed but most economic setting is still 8 or 9 knots
better batteries are always good, they have no real downside
the planes off Gibraltar, well yes be careful of those but don't go completely paranoid, you can still reasonably safely surface at night
as to gas stations along the way, depending on which mods you're using, there's 2 or 3 in neutral spain and one in the canarie islands, not in the stock game though.
If you're playing stock I suggest heading towards Freetown but then making a sharp left just past Spain and loiter off Gibraltar, lots of shipping there, taskforces with capital ships included
Good hunting
