Double Din replacement options and advice

[In the sticks]

Hi fellow owners. I'm new to the forum having just purchased an 04 C4S in cobalt blue. (have posted this in 911UK as well but hopefully adding further views by posting here as well)

I need to upgrade the stereo and have purchased an Alpine 705 (didn't want the PCCM plus due to a number of factors - I hope I do not regret this)

I have a couple of questions on the install options and I'd be really interested to get your views on them.

So, I already have the PCM 2, so double DIN. Whilst it is not BOSE, it does have the amp by the spare tyre and this has MOST/fibre.

So, options are to get a convertor to use the existing AMP, circa £200 ish. or replace the amp with something else.

I don't want to spend a fortune as I know that the speakers aren't brilliant so I'd be really interested to hear what thoughts, ideas, products, advise you may have.

Thanks so much in advance and I hope this thread helps others who may also be looking at this.