Others will comment on the state of the local market and insurance, but considering you can still register imports i would bring in something from Japan. Also, considering your mechanical experience an engine or gearbox swap will be easier than body work too, so go on the hunt for automatic non-turbo examples
Upon arrival a full under sealing and rebushing job will teach you how everything works and get it ready for the conditions. That way you'll have a solid base
People will look at you as though you're mad for importing a non-turbo automatic 200sx from Japan, but the flip side is that after a manual conversion you have a car that hasn't been exposed to UK roads and has had a higher chance of not being neglected. S13's in good condition are only going up in value. Not to say try make it pristine, but starting from an 8/10 base is going to cause 100x less issues than a 3/10 example. There are plenty of examples of SR20DE's being used on here so scour the build threads
If everything works out after that then you're only looking at a set of coilovers, a welded diff and a bucket seat to get out and have fun. After those are covered off look into a 4.3 diff (assuming it's your daily so no crazy 4.6 ratio), rack spacers and a nice steering wheel. No point in engine modifications tbh, just keep it serviced. Don't go crazy on all the adjustable suspension arms or fit 18x10 wheels on it, all of that is money that could go into track day passes, petrol and tyres