Any file conversion will result in at least some loss of quality, and as a DVD-V only uses MPEG-2 video it will need to be converted.
However as a general rule 1 hour of video onto a SL DVD should result in minimal losses. Depending on the quality of the source video you may be able to go to around 2 hours, ie from VHS you could probably get more onto a disc before the quality loss becomes noticable.
Within many DVD authoring programs you will have options such as doing a multipass conversion that can help to maintain quality, but at the cost of conversion time. Also settings such as VBR (variable bit rate) rather than CBR (constant bit rate) can help to give the bit rate to more complicated sections and reduce it when its less required. It is also worth noting that not all MPEG-2 converters are equal, so some programs will give better results than some others.
What version of Windows do you have? If Vista or 7 then there is a DVD authoring tool built in to it called Windows DVD Maker. If the results from that are good enough then no need to buy anything, but if not then you have the likes of Nero etc to look at.
Mark.