Node.js + MongoDB + AWS engineer for SaaS acquisition: code audit, then infrastructure migration

Node.js + MongoDB + AWS engineer needed for a SaaS acquisition — technical review first, then infrastructure migration. I'm acquiring a small subscription SaaS business and need an experienced backend engineer for two things: a fast technical review before I close, and the infrastructure migration afterwards. The product is a live, revenue-generating application with several hundred paying subscribers. It has to keep running throughout. This is not a greenfield build — it's careful work on someone else's production system, with real customers and real recurring billing attached to it. STACK Node.js · MongoDB (Atlas) · AWS · third-party LLM API · third-party subscription billing platform with webhooks PHASE 1 — TECHNICAL DUE DILIGENCE (starts immediately, roughly 8–15 hours) I have a short diligence window and need an independent read on what I'm buying. - Review the codebase: quality, structure, technical debt, anything alarming - Identify security issues — exposed secrets, injection risks, weak auth, misconfigured storage - Inventory the infrastructure: what's running, where, and what it costs - Verify the third-party integrations, especially the billing webhooks — confirm exactly how subscription events reach the app and what would break them - Confirm whether the MongoDB Atlas organisation, and the prepaid credits attached to it, can transfer to a new owner - Check third-party API usage against those providers' terms of service - Flag anything that makes the migration harder or riskier than it looks Deliverable: a written findings document — plain language, prioritised, with a clear statement of anything that should stop or reprice the purchase. I would rather hear bad news now than after I've paid. PHASE 2 — MIGRATION Move everything from the seller's accounts into mine, with minimal downtime and zero data loss. - Migrate the AWS infrastructure to my account, or rebuild it cleanly there — tell me which you recommend and why - Migrate or transfer the MongoDB Atlas organisation, preserving the data and the prepaid credits - Move the codebase into my repositories with full history - Transfer the domain and cut over DNS - Re-issue every credential and API key in my name, and verify the seller's access is fully revoked at the end - Keep the subscription billing integration working without interruption. This is the single most important requirement — the recurring subscriptions are the asset I'm buying. Webhooks must never silently stop firing - Set up basic monitoring and alerting so I know if something breaks - Write short handover documentation: how to deploy, where things live, what to do when it breaks The seller is providing up to 20 hours of engineering support for the handover, so you'll work directly with their developer rather than reverse-engineering this alone. Deliverable: the application running entirely on my infrastructure, verified working, billing intact, with documentation I can actually follow. PHASE 3 — ONGOING (hourly, optional) Ad-hoc support for issues that surface after migration. No commitment either way. WHAT I'M LOOKING FOR - Solid production experience with Node.js, MongoDB and AWS — not tutorial level - You have migrated a live system between owners or accounts before, and can tell me what went wrong last time - You understand webhook-driven subscription billing and why a missed webhook is a serious problem - Comfortable working inside a codebase you didn't write, without rewriting it - Clear written English — I need to understand your findings, not just trust them - Available to start within a few days and responsive during the migration window Nice to have: DevOps depth (IaC, CI/CD, monitoring), experience with subscription billing platforms, prior work on an acquisition or handover. WORKING CONDITIONS - You'll sign an NDA before receiving any specifics or access - Production customer data stays on managed infrastructure — no copying databases to your local machine - Credentials shared through a password manager, never over chat, and rotated at handover - All access revoked on completion TO APPLY Please don't send a generic proposal — I'll read the first two lines and stop. Answer the screening questions and tell me about one migration you have actually done, including what went wrong.