[petrochemist]

I think your best approach is probably to get a cheaper camera now & upgrade later. Cameras are advancing so rapidly at the moment that a top of the range model will feel dated in a years time. I also found that using my second hand 6MP DSLR for a year got me to realize which features I valued most helping in selecting the right upgrade. For many subjects the picture quality with my older camera is visually every bit as good as my newer 14MP job, and in one obscure field Infra red shots) it is considerably better so I've ended up keeping & still use the old one sometimes.

You are quite right that many accessories will keep you tied into a particular brand - there are some adapters available but for lenses t least more advanced features are lost if working via an adapter. At least tripods are pretty much universal fit.

I believe that you'd have to look hard to find a modern DSLR that's not capable of taking the sort of shots you're after, so perhaps the most important aspects are how it feels in your hand & how YOU get on with the menus etc. They'll all have macro & long telephoto lenses. You'd have to have very expensive lenses to stand a realistic chance of seeing a real improvement above 16MP. Low noise at high ISO, fast auto-focus & image stabilization will all be useful.

The sort of photography you're interested in is challenging, requiring both skill and patience. The techniques can be learnt perfectly well with slightly older models, allowing the new hardware to develop further (& prices drop) before you upgrade. Watch out though it's tempting to upgrade... 2 months after I upgraded the model I really wanted dropped in price to what I paid for the one below it.

The general opinion seems to be that quality lenses make a huge difference. I've not been able to afford the very best, but I've had some excellent results from some very cheap lenses (<£50), unfortunatly a few others have been so bad I wouldn't dream of trying to sell them.