My plan (which I've done before) is to wait until my existing tyres nearly need changing on the original alloys.
Then instead of paying around £300/£400 for a full set of new tyres I buy a cheap replica set of alloys/tyres for around £599 and keep my original alloys and tyres (just legal).
Then you have a spare set of alloys for the difference - around £199/£299. Always keep your originals for when you sell and if they need touching up you can do this when you get the replicas on.