You can always negotiate on price, but in my experience housebuilders wont drop the price - they do however offer other incentives: carpets, garden done, free solicitors, stamp duty paid, removals paid, cash back.......there are alot more incentives for house-owners (some buy the previous house and pay those costs too), as you wont want any of that I would push for more discounts.
As for time taken, its likely to take 6-8 weeks as per a normal buy.
Also make an offer, ours was marketed at 465k we paid 430k. Is the house built or are you buying off plan, if the house is ready to go should take 6-12 weeks, depending on solicitors pulling their fingers out. If off plan you will need to find out the completion date, which in my opinion is always delayed. If you are in rented you are a more attractive buyer to a developer so be cheeky on the first offer, you can always up it. Our was really cheeky, but they accepted first offer as we were in a position to proceed immediatley.
reservation fees will vary, you need to ask the developer. Good point about whether the house is already built, if it is finished they will be more desperate to sell it, so you can offer less. Even if they refuse a discount on the sale price, ask for other discounts or freebies as I said above.
Ours was £500 for the reservation fee. Did have any idea we could negotiate on asking price so we paid full asking price. I think they had offers in most of the properties but lots of people's mortgages fell through. The last person to buy paid 20,000 less than us I was fuming when we found out. Was fairly quick and easy to complete the sale. Wish we hadn't bough a new build though.