Hello Martin,
I can share my experience with you, I started to work with 8051 several years ago. I just went to buy a Atmel AT89c51 chip and following the document found in Atmel's website to buid a simple programmer. Then I found an old Intel A51.EXE, L51.EXE and started my work with a PC that running MSDOS6.21 and then later I moved my work to Win98(MSDOS prompt). I just need to found the datasheeths of 8051 together with the instruction sets. Then doing 8051 is just similar with the old Z80/6502/8088 .....etc. To get the things smooth, I suggest you to sit down and to study the 51 core carefully, pay attention on the programming model, what resources that can be found in your chip..etc. Well after doing several projects, you will get familiar with this chip.
Finally hope you get smooth with your projects.
Best 73 from Li.

P.S. For the ATmel 89C51 chip, those older one (before 1998) that cannot secure the internal EEPROM data is very very cheap indeed. Those old chips are useless in commerical applications but still useful for learning purpose.