Izzirent is a broker who (most likely) uses tour operator rates. They usually have unlimited mileage - if not it's stated prominently right in the quote. To make sure, call or email them and ask. Especially seeing you have no history with Sixt at all, it's highly unlikely they'll match the izzirent price when booking direct.
At Sixt, the outlets in different countries are separate entities who can set their own prices to a degree. There's no real logic, but at least the internet has made it real easy to shop around. I usually use the German site (sixt.de) which shows a price of 143 Euros for the cheapest car with unlimited mileage and CDW (500 Euro deductible) for your location and time.
It's absolutely possible to return the car even when the station is closed (key drop box). You only need to mention that when you pick it up.

practical advice:
Depending on how much you'd want/need to drive, see that you get a diesel car. Diesel is about 30 cents/liter cheaper than gasoline in NL plus fuel economy is about a third better, so even though you'd be paying a little bit more for a higher car class (the smallest car group doesn't have diesels) you'll end up spending less overall.
How old are you? As you say this is your first time renting a car, check the minimum age and drivers' license requirements (21 and two years, respectively - higher for larger cars).

When you need rental cars for business often, get a corporate account with Sixt. In addition to discounts, this also expedites the rental procedure. Although I have found tour operator rates from portals like izzyrent or billiger-mietwagen.de even beat corporate discounts in most cases, especially for rentals of three days and longer with insurance added.