yeah defo gonna go with get the modifications first then a custom remap. decided gonna take peoples advice on here and start with the intake and boost hoses then ill look to move onto a hybrid k03s or a k04 with either a front mount or an uprated side mount.
Wouldn't bother with an 'updated' side mount, you will always still have the restricting factor of pish air flow through it no matter how large it is 
i was looking at the forge side mount and there was another slightly cheaper option but front mounts are gonna spank my budget straight away thats why i was thinking sidemount but ideally im gonna want to get as close to 300bhp as possible so chances are im gonna have to bite the bullet and go front mount.
Without sounding rude

your not going to get anywhere near 300bhp from £500 fine ones

A good stage 2+ spec
Welly cooler DIY Fmic -£280
Forge Tip £130
Forge 007p £85
Forge actuator £125
Cai £80
3" performance exhaust inc decat £250
Devils own water methanol kit £300 (optional for extra tuning potential/cooling factors)
Integrated forged rods £300
Hybrid k03s £450+
Badger 5 ported manifold £80 + manifold expense£
Rtech custom remap £300
Ball park figures but that's the sort of spec you'll be looking at for breaking the big 300