There are a few routes you could take, and the main decision points are budget and retention of the stock head unit. As a starting point, the basics you can do either way are to replace the door speakers and add sound deadening (foil-backed butyl and closed-cell foam in the doors). To get the best out of your new speakers, you'll want an amp. If you're keeping the stock HU, you'll also need a line level converter or an amp that'll take line-level input. The converter is probably the easiest, and you can get something simple like this VIbe thing for £20 or something a lot more advanced like this LC1i for nearly £100. Both take input from the stock speaker wires, so there's a bit of tinkering to do with the wiring loom to tap into them. Once installed, you have RCA connections for an amp. If you're replacing the HU, you don't need the line level converter as the new HU will have RCA jacks on the back - just don't get a cheap aliexpress thing, go for JVC, Alpine, Sony etc. The pitfalls of doing this are that you lose the car config menus in the standard HU - I was ok with that as I only have a basic spec 'SE' and everything I could do on the HU can be done with a physical button in the car, so I lost nothing really. At this point, amp choice is largely down to budget and need. There is no denying that an amp with a DSP will allow you to get a better sound out of your gear, but you have to know how to use it. It's not a plug and play kind of thing unless you spend big bucks on something like a Helix with auto-tuning functions; you need to measure the sound with a calibrated mic and then learn how to use REW, which isn't too hard but it does take time to get to grips with. I have a Vibe Powerbox 65.4-8DSP, which is the real budget end of these things, but does work well. I had the non-DSP version before this, it's about half the price and a decent little unit. Sub/Amp again comes down to budget, and also the music you listen to in some respects. If you don't listen to music with a lot of sub-bass, you probably don't need a sub. Generally, you'll be able to daisy-chain the amps together, with your 4 channel having an RCA output to send signal to a mono amp for the sub. Pick your sub based on budget, find it's RMS power and Ohm value then find a mono amp that delivers slightly more RMS than you need at the same Ohms as your sub. That means you'll have clean signal headroom, and shouldn't need to stress the amp to drive your sub. For both amps, you'll need a wiring kit and some distribution blocks to split power between them. It's all DIY-able if you take your time and do your research. YouTube is your friend! For shops in the NE, you have AutoSounds and Sounds Alarming, but the cost of a good install might surprise you, it takes a long time to do it right and their hourly rates aren't cheap. You say you've spent money with local shops already - what have you bought?