with an $800+ budget you can certainly get off to a good start.
The Nikon D5000, and D90 are good places to start. Additionally I always enjoyed the Canon 10D-50d series
I remember when I started with a Pentax k10d and a Quantaray 70-300 macro (together they cost about $300 now)
The Quantaray was a rebranded Tamron essentially and I still came out with decent picts like these. For the price and my inexperience, I found the combo worked quite well.
Pentax K10d + Quantaray 70-300
My bad focusing this one on the nose!
But there are plenty of other cameras that can still be had for a bargain.
The Pentax KX, KR, K20d, or K7 can all be had for under $800 easily
THe K20d in particular sells for about 400 used right now and I have two that still perform great!
I've been using the Sigma 105mm macro and the Pentax DA 35mm macro (both are sharp but the Pentax is sharper)
I've gotten great results shooting Fish with the 105mm macro and Corals with the 35mm macro.
Pentax K20d + Sigma 105mm
The K20d, K7, and K5 are weather sealed and can be had with several weather-sealed lenses (DFA 105 macro F2.8, DA*16-50 F2.8, DA*50-135 F2.8, DA*55 F1.4, DA* 200 F2.8, DA* 300 F4, and DA* 60-250 F4 )
I have the DA* 16-50, DA* 50-135, and DA* 300 and have used all of them extensively in rain with no issues.
Another bonus is the backwards compatibility with most pentax MF glass. I've been using a Pentax M 50mm 1.7 that I got for $20 at a garage sale.
Downsides of older pentax models are sluggish AF tracking and low light performance (Two things that were conquered in the new pentax K5)
Esentially I'd suggest purchasing used gear first to gain experience and invest in good glass. Whichever brand you choose there should be something that will work well for you.