£3.5k max budget — how close can I get to ChatGPT with a local AI workstation?

I'm looking to build a serious local AI workstation, with an absolute maximum budget of £3,500.

My benchmark is ChatGPT Plus. I use it heavily for professional work: deep research, analysing PDFs/images, producing reports, PowerPoints, strategy documents and generally turning rough briefs into polished deliverables.

I'm happy for local inference to be significantly slower. Quality matters far more than tokens/sec.

What I'm ultimately trying to build is a self-hosted AI work assistant, rather than just a local chatbot. Ideally:

- Strong reasoning/writing approaching current frontier models

- Large context + local document/RAG access

- Vision/PDF/image analysis

- Web search and multi-stage research

- Agent/tool use to create PPTX, DOCX, PDF, spreadsheets etc.

- Persistent project knowledge/memory

- Remote access — ideally I could send it a task via WhatsApp/Telegram/email and have the finished files returned to me

- Local/offline inference wherever practical

Essentially, I'd like to remotely send:

"Research X, use my project files for context, investigate current public information, then produce a detailed report and 10-slide presentation."

…and let the machine work on it asynchronously.

With £3,500 maximum, what hardware + model stack would you build today?

I'm particularly interested in whether I should prioritise maximum VRAM via used 3090s/multi-GPU, a newer single NVIDIA GPU, high-memory Apple Silicon, or something else entirely.

And which current open-weight models/quantisations actually come closest to ChatGPT/Claude quality for long-form professional knowledge work?

I'm technically comfortable setting everything up, so Ollama/llama.cpp/vLLM, Docker, RAG, agent frameworks etc. aren't an issue.

I'm primarily interested in what £3.5k buys me in real world capability, and whether what I'm describing is actually achievable locally yet, or whether I'd be spending £3.5k to build something noticeably inferior to a £20/month ChatGPT subscription.

But having said that, being able to build something that if it works, will remain working indefinitely is highly appealing to me.