I can speak some about cameras and lenses and how they work:
you'll notice that manufacturers generally have 2 different priced lenses that are roughly the same at first glance, same focal length. However they vary greatly in price, one being on the order of 3-4x more expensive. The cheaper one is meant for APS frames and the more expensive one is meant for full frames but can certainly be used on APS camera bodies(of the same manufacturer), you really don't want to use an APS lens on a full frame camera the image that it paints on the sensor is not going to be as high a quality on like 40% of the overall image. The cheaper lenses have a much smaller area of the lens that has to be ground well enough to meet QC standards that's one portion of why they are cheaper. Generally the more expensive ones will have 2-5 more pieces of glass (correcting chromatic and spacial distortions) in them as well, the big thing you will notice is the F-Stop will go much lower on the expensive lens.
Link:
sensor sizes comparison
Each click of the F-stop is double/ or half the amount of light coming in, meaning it is opening the iris that much more,
A pinhole camera (Fstop of >32) has an indefinite depth of field. However a very "fast" lens like one that goes down to an F stop of 1.8 when wide open has a very small depth of field. that's how all the wedding photographers get that fuzzy background when they take portraits. They open that sucker wide up and even at a distance of 5-6 feet from the target theyre lucky to get 5-6" of depth of field where the focus is perfect. All things being proportional you hopefully get the drift.
Generally when tying to capture the perfect shot you need the following
*Proper depth of field
*Proper Lighting
*Proper exposure time (if the object is moving even slightly you dont want blur)
*Low Sensor Noise (Ideally a camera takes the chromatically most accurate shot with the sensor at unity gain depending on the manufacturer that is 1 or 0. )
What you end up with is a giant compromise because a number of those require opposing environmental inputs or are cost prohibitive to solve.
you fight not having enough light (probably the most expensive to correct) by some combination of:
*opening the F-Stop - reduces depth of field
*increasing the exposure time - can cause blurriness do to either hand shake or object moving
*increasing analog and digital gain of the camera sensor - lowers the accuracy of the color capture
Regarding pixel density CMOS sensors have really come a long way in the last 10 or so years and you can pack a lot more pixels in the same area, however that requires more light as pixel thats half the size in both X-Y is only being hit by a 1/4 of the light. so they are noisier. Theres a lot of color correction going on in a DSLR camera.
End Summary:
if you choose a Full frame camera body even used, youre going to want to have full frame lenses which will end up costing you tons. if you choose a newer APS camera body that has a metric buttload of error correction you can still get beautiful photos and still splurge on a lens if you really want a nice one however the total size of the image will be a bit smaller meaning a 28mm lens on an APS is closer will capture the same image as a 35mm on a full frame if the pixel count is equal at roughly the same distance. however if you slap a 35mm full frame lens on a APS camera its going to be a smaller FOV. If you slap a APS lens on a full frame camera theres going to be some aberrations on the outer edges of the captured image.
Generally lenses will hold their value quite well and bodies wont. The newer tech on the bodies can provide significantly better shots meaning a newer APS might be a better purchase than an older Full frame costs being equal.
Clive will probably soon tell you that doing a color correction for the environment that youre taking the shot will fix off colors and he's correct. you need a target thats big enough to fit the entire FoV.