Creosote is readily available. I just added diesel by eye ... a guesstimate you might say.
I used two barrels and each held about 12 posts .. the more posts you get in the higher up the post your creosote/diesel mix will travel.
I recycled the creosote from the barrels using an old jug and some 25 litre drums. I have it stored for painting on the posts/rails when needed.
By my reckoning, you'll need 42 posts and 21 rails per run (total 42 if a double rail and 63 triple rail).
Two schools of thought re. standing the posts ... one where you just embed them in gravel and pack tightly ... water soaks away from end of post so less chance of them rotting in the long term; two where they are embed in concrete ... but a higher chance of them rotting.
I myself, have used both methods for two different reasons. I dug the holes myself and just used gravel and they are still standing proud today. Another property I have ... the hole were dug with a hymac so were too big for just gravel alone so I had to use a gravel/concrete mix.
The €850 for the posts & rails supplied sounds ok. If the professional has help he should be done and dusted in one day. If you do it yourself with help, give yourself 3 days ... all going well.
Re. your question about soaking and painting afterwards ... that will be a perfect job.
The most important thing is either put caps on the posts or cut a 45 degree piece off the top (facing the back) ... that's where the rot will start. I made caps for mine and every so often I'll replace one that has rotted. Posts are still perfect to this day. Initial ones were put in approx 10 years ago and with care will last.
I can't suggest people here but if you post in the correct forum I'll answer you there:
http://www.askaboutmoney.com/forumdisplay.php?f=74