[ajj]

If your objective is only to get focused and motivated, and not necessarily do research, a PhD might be too much of a time sink (I am a PhD student in CS). More so if you are thinking of taking it up part time, since most of your free time will be spent on it. Further, expect that the first couple years will not give you many positive returns to motivate you to go on: those are mainly spent building the foundation.

Having said that, if you truly enjoy the process of research, that might be just the thing to get you more focused and motivated. I definitely enjoy it at the moment.

In summary, don't go for it only to get motivated - if it doesn't interest you, it might end up taking too much energy for nothing.

[throwaway1]

I enjoying learning and discovering new things. My main motivation is to produce an original piece of work - something that will define me, if you will. A magnus opus.

Is it a waste of time that after two years of part-time research you find that the phD is not the approach for you and you abandon it?

The finding it interesting bit, is another questions in the back of my mind. Will this be something I will find interesting enough to keep me going for the next few years.

[ajj]

"My main motivation is to produce an original piece of work"

In that case, I suggest working on it right now, before a PhD. You can start doing a heavy literature review on a focused topic, and try to improve the state of the art by however small you can. The standards for publication are much lower than what many people outside academia believe.

You can do this and even get a publication without an advisor (may be not at a premier venue but a decent one nonetheless). That gives a great sense of accomplishment.

If you want to continue, the work will surely help you in your PhD (won't be wasted time). If you don't like it, you can move on to other things (and it won't feel like giving up on a long commitment).

The reason I say this: the beginning of a PhD is probably the hardest time to see results, and keep you motivated. So starting small would be helpful. Once you believe you need larger goals and are not satisfied by the small accomplishments, you can go for the long haul!