Hi Arturo
For garden photography it sounds like an excellent plan to keep using your older lenses. In fact there are still a few manual lenses manufactured and sold today. If your lenses are from the 80's you should probably be okay using them on a modern DSLR body, but why not check compatibility and what functions will work on which body here:
http://www.kenrockwell.com/nik...
or here:
https://support.nikonusa.com/a...
Another question would be if you require mirror lock up or not, as many entry level Nikon cameras unfortunately lacks this. I believe that as a workaround one can use live view on some of the newer cameras, but I've read different stories here, so check carefully. My D300 while having mirror lock up, doesn't support it in live view, but this is an older camera.
Personally I don't care much if the camera has 10 or 20 megapixels as the difference is much smaller than the numbers would suggest, so provided you can find something second hand of good quality, I think it could be a good money saving option. With good glass you will get excellent results regardless of the number of pixels! If you want to shoot in the dark hand held then of course a newer full frame camera would be your best option, but for lower ISO images, (which I assume is what you are using today anyway) almost any older DSLR will work well. Probably you will still want to check a lot of reviews on cameras and DpReview probably is the best source for pixel peeping:
http://www.dpreview.com/produc...
I think your biggest challenge would be to decide if you want a full frame DSLR or if you are okay with the smaller and cheaper APS-C sensor cameras. If you decide to go for an APS-C sensor camera it's important that you understand the differences as your lenses won't behave as they do on your camera today. In stead of explaining the difference myself I just link to what I though was an easy to understand guide which explains it much better than I would be able to anyway:
http://www.mdavid.com.au/photo... . Another factor to consider would be the size of the viewfinder as this usually will be smaller on an APS-C sensor camera.
BTW Personally I have never used a film camera extensively, so as always do your own research and good luck with your choices.
