erm, to do what you want to do im not toooo sure to be honest, i think you could get alot of power from both engines to be fair, but for starters just by using the 1.8 your going to need alot of custom work for mounts etc, so just by doing that your spending alot already and you havnt really gained much by it
but it depends what you want to run, would you go boost or n/a
with a good set up on n/a you cold probs get a good 190bhp on the vts engine and boost around 240bhp is a figure ive seen quite often, as for the xantia engine im not too sure, but if your doing alll that and really want to do it, i think your best using say the xsara vts engine or gti-6 as they are same, 2l 16v 167bhp, it will fit with a struggle and be alot of custom work same as the 1.8 but if you really want to run a bigger engine i would go for the 2L personally
but persoanly i would run n/a with a vts engine, this is what i plan for the future, as my saxo will be a track car when i get the funds to run that engine set up lol
and most others will agree that just going vts will still be a great set up and more than adiquite if modified in the right ways and if you have deep pockets lol
hope this helps