The GlobalSat BU - 353 - S4 works as simple as it gets, by plugging into any USB port and selecting its COM port from GRLevel3 in the GPS menu. Machines not running Windows 10 (8, 7, XP, etc) might have to install a driver which is easy to find and install.
I've used the aforementioned model of receiver for 8-9 years so can vouch that it is reliable and rugged.
As for maps: GR3 shapefiles are exceptionally detailed with the only drawback that names are not shown on each road unless you mouse over (has never bothered me). The roads files have every dirt patch and side road that existed up to the year the shapefile was made. New census data updates them every few years. I am able to navigate almost entirely with GR3, although I run my car nav screen or google maps as a second view.
If you want to use roads shapefiles from within GR3, then adding shapefiles , in my opinion, is best done by state or radar site, not county. I'm not sure what benefit there could be to doing that on a modern computer but we each have our ways of working. With several statewide roads shapefiles loaded, almost any computer made within 3 years will have only a very slight slowdown when starting GR3 or switching radars, but we are talking seconds here (5 seconds average, or even less). Not AT ALL worth spending HOURS loading these by county for a couple seconds of perceived lag when starting up or changing radar sites to another state, but maybe that's just my opinion. I'm using a middle range Surface Pro 7 here with an i5 processor and 8GB of RAM, nothing fancy at all and I never have any lag or issues when loading roads by state into GR3. My previous extremely underpowered netbook was also able to handle loading roads by state, but the starting of GR3 or changing radar sites to another state could sometimes take 20-30 seconds (especially huge states like TX); again that netbook was very weak and had Windows 7 on only 2GB of RAM and a slooow processor. For any modern machine, typical delay of is going to be 5-10 seconds or better only at startup and when changing radar sites to another state- not something that has ever bothered me chasing. The files also are not large compared to RAM on most current machines. Almost all states shapefile size are between 100 and 220 MB. You can load up a few states before a chase in ~two minutes. No noticeable performance issues at all.
Note: road shape files only show up when you select a radar that covers the road area. So, for example, you cannot browse the detailed Kansas road networks shapefiles in GR3 with a Colorado radar selected.
