Hi
Buer Hassel are 2 districts - one called Buer, the other just North of it is Hassel. They're located in the north of the somewhat sizeable city of Gelsenkirchen, located some 20km WNW of Dortmund, and about 40km East of the Germany/Netherlands border. It's pretty much in the centre of this grab of Google Maps I took: https://www.google.co.uk/maps/place/Bottrop/@51.5927021,7.0540388,12z/data=!4m2!3m1!1s0x47b8ebd6912e5e2f:0x9dee8340d56bd0e0
If that link doesn't work, search for SC Buer-Hassel 1919 e.V., on Google Maps (which is the local football/soccer club - their website is http://www.sc-hassel1919.de although it likely holds little of interest to you). Of perhaps more interest is the Wikipedia entry for Gelsenkirchen as well as the Wikipedia entry for Bottrop, which are the 2 cities that kinda border Buer Hassel to the SE and SW respectively. The Wikipedia entries have some fairly high level stuff about what happened to them during the second world war.
Kirchhellen is about 5km to the West of Hassel. Head for the extremely obvious power station and you're about half way there to Kirchhellen.
As for Margraten, details of how to get there are available at http://www.abmc.gov/cemeteries-memorials/europe/netherlands-american-cemetery .
To be honest, I wouldn't bother hiring a car except for a day to drive around Buer, Hassel and Kirchhellen (and only then to avoid getting confused by local bus services). Public transport in this part of the world is just too good - when you're in this densely populated an area. Refer to http://www.bahn.co.uk (which is the English language version of the German rail system) for a full train + bus + tram timetable which will tell you how to get to wherever you're going whenever you're going there. Basically, from Amsterdam airport (which I assume you fly into), take a train to Amsterdam Centraal, then from there you can head down into Germany, or direct to Maastricht if you want to visit his grave first. If you do hire a car, get a sat nav.
The sightseeing options are numerous. Refer to http://www.ruhr-tourismus.de/en.html which is the official tourist webpage for this area. The Landschaftspark is quite something, as is the surprisingly interesting Bergbau museum, all about mining which dominated this area.
As an alternative to flying into Amsterdam, consider investigating flying into Duesseldorf airport which is much closer to the region in which he was killed than Amsterdam is. I can't really help with hotels or restaurants (I live at the other end of the Ruhr valley some 80 odd miles away so am not that locally familiar). Tripadvisor and Yelp are quite handy for identifying good places to stay and eat.
As for finding a road map of Kirchhellen for that time of year, dunno, that's stumped me. Perhaps you could try emailing stadtbibliothek@gelsenkirchen.de (try using Google Translate if you don't speak German and can't find anyone who does) which is the local library in Gelsenkirchen. They may not have one on file, but maybe know someone who does. In the interim, I will ask around if anyone I know has any idea.
One note of caution - Gelsenkirchen suffered particularly severe damage in world war 2 because it was home to a hydrogenation plant at Gelsenkirchen-Scholven (which is extremely close - a mile or two) to where you said your Dad was killed. Because it produced gasoline this was of particular importance for air strikes, maybe your Dad was involved in trying to capture/destroy it at the time. If he was in the USAF, http://en.wikipedia.org/wiki/Oil_Campaign_chronology_of_World_War_II says they attacked it on March 8th 1945 with the RAF following up again 2 days later. To get an idea of the damage caused to the area by Allied attacks, here's a picture of the plant 2 months dated May 1945 as taken by the USAF: http://www.spiegel.de/fotostrecke/photo-gallery-documenting-destruction-fotostrecke-54868-32.html . As you'll appreciate, there's little trace left today of what it looked like, nearly 70 years later. As a result of such damage this may cause you problems specifically with road layout - it may well have substantially changed from 1945's layout. Still, I'll try to find out where on earth you could get a copy of a map of the area (I have no idea).