No, 12k is too much for a hobby unless you are a lottery winner. I was in the same spot a few years ago, here are my thoughts.
I thought the 100-400 was too pricey, so I went on safari with a 70-300 IS. I learnt from that and soon bought a 100-400. I figured it was the be all and end all, that was alot of money for me back then. It went with me to India and Galapagos. It is sharp but was less useful in low light especially with my 5Dc. Eventually I bought a Sigma 500 f4.5, I was stoked, the Euro was strong against the pound so I made a great deal. In short, the 100-400 took better pictures, focussed faster and more consitently. So I sold the 500, made a pretty good profit having imported from the UK. I had enough to get a 300 2.8 IS used, pound was still low, buy in the UK. Got me some TCs and WOW. I was impressed. Used that baby for 2 years but found for most of my stuff (here in the forests) I need 600mm most of the time. So then I came to the 500 f4 IS, once I thought it was for lottery winners only, now I found myself in need. So I bought one used, cost me 4100€. I sold the 300. Each time each lens cost a little bit more, so it wasn't such a shock to the system.
Now I have the 500 and cannot be happier. Current used prices are higher than what I paid, so it has not cost me a penny to use it for 2 years, if fact I could make a profit if i sell. But I wont, yet.
Basically what I am saying is, try and find a good used copy and it will not cost you anything providing nothing goes wrong. It is an investment. I am selling shots now, because I have better IQ and sharp shots, so my lenses cost nothing but make money.... cool!
If you are worried about the money buy used, look at the 500 f4. With a TC it gives you 700mm f5.6 and is sharp wide open. It is also far lighter than the 800,400 or 600 options (unless you go mkII). But buy what you need first time, saves all that selling and wondering "what if". The white lenses wont lose value.
