The things that are important to me are (in order of importance):
-easy to paddle
-easy to catch waves
-able to progress and do some turns
-durable
-cheap
I am 170cm/5'7" and 60kg. My nearest beach is 1.5 hours drive away (UK North Sea) and I'd surf 1-3 times a month. The waves there are wind swell mostly, 1-4ft, 5-6s period with fairly strong onshore wind is typical. 8-9s period is about as long as it gets, I think. It's a beach break.
I've done about 5 hours surfing, and in the last 2 hour session, I have paddled out and caught 3 unbroken waves and rode them in straight (nose dived or caught, but fell off many more). I was so happy about the long ride I got that I didn't even think about trying to turn

I'll rent the foamie a couple more times first, but I was thinking maybe a hard long board would be good choice for me as a first purchase. I have no desire to work my way down to a smaller board - I like the laid back long board style and it seems more functional for the conditions I have. (Is that right?)
So, my question: I know people say if in doubt, go for longer/bigger, but are there any disadvantages to having a bigger length/greater volume board? Is there such a thing as too big? I was considering an 8'6" x 22.5" x 3.125", 67.2L (http://www.torq-surfboards.com/long-86.html). Is that too much volume for 60kg? My paddling speed and endurance is not that great, and likely never will be, so I figure I need all the help I can get.
Would a lower volume board be any easier to get out to the back? Since having one small enough to duck-dive isn't an option for me, I'm thinking maybe it wouldn't make much difference? I'll have to turtle roll through anything big anyway, so I thought any extra float to help paddling would be a good thing.
Would this much volume make turning too hard? From videos I've seen, it seems that with the right technique, even small people can turn big boards.
Various volume calculators seem to think I should have half this volume (even at the most extreme beginner end of the spectrum!). I figure they are meant for more of a shortboard style of board, but wanted to check before I go against them all

Any other comments/recommendations gladly received!