It's very easy to install a CF HDD into an A1200, I prepared a set of instructions that I supply with my kits:
And installing the Ram/Accelerator is easy, you just open the trapdoor and it slides onto the edge connector, some of the larger cards can be tricky so you can open the A1200 to install like this:
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(ideally you should be taking anti static precautions, normally I work on an anti static mat with wrist band (earthed)
The A1200 you liked is a fair price in today's market and would make a good base to upgrade, normally they will be supplied with Workbench Disks but you can also buy a set at Amigakit, Vesalia or eBay. I might even have a spare WB3.0 set I can include.