First, stop by an ANCAP gas station and pick up a great map of the country and main cities. It will cost you about 4 bucks and be well worth it.The driving conditions are quite nice, once you get out of Montevideo (and really not bad in the city compared to some places). You will find the rural speed limits to be 90 to 110 (kilometers per hour -- around 60 mph).
Trucks seem to have a lower speed limit and seem to obey whatever their limits are. Cars will speed. A problem area is when the speed limit reduces from 110 to 75 without any advance warning. If you actually slow down you may have someone way too close.
Not sure about the ample paved shoulders for bikes. One of the warnings we got about a driving trip after dark was to watch out for bicyclists.
There were several bicyclists on the highway without lights or reflectors in the rural areas! When we got close to Montevideo (but outside of town) there was plenty of them (without reflectors or lights). AFTER DARK!
Once you get out of Montevideo you have mostly sparsely populated farm land.
Someone else might have more info.