I hate being told NO, even when nicely packaged.
So re the business plan: value of providing expertise/spending time helping everyone elses bespoke business (ie. MSP) is greater than value you’d be providing in building PCs for those businesses (they can get a prebuilt off a factory line from HP or Dell or Lenovo and call it a day).
To become an expert you need time, to be able to sell expertise you need some form of reputation or you need to undercut the competition by enough for someone to be willing to risk what they need for their business on you.
I’d maybe try and come up with 3-5 similar products, rough sketches (e.g. setting up PCI compliant wifi, offering a service plan on older and cheaper hardware, email, backups). … and figure out solutions, total addressible market in the area you want to service, figure out pricing.
Don’t assume you’ll get the jobs because you’re better or cheaper than anyone else.
Don’t assume long term contracts, make some drafts that makes it easy for future customers to use your services and where if they want to break the contract and pull out, both your losses are minimized - no big irrecoverable investments.
Assume you’ll get 1/1000 jobs in your total addressible market in the first 6 months in the beginning, and build from there.
Use that to come up with cash flow projections over time.
Additionally, see what other stupid jobs you can get and hold (flipping burgers, or door dash and whatever), that are compatible with this one, so you have something to fall back onto, if your own business crashes and burns (it happens more often than success happens).
Don’t get into debt that’s bigger than 2 months of your “flipping burgers” income e.g. don’t rent fancy space or buy a new car or even a second hand fancy car, SUV or whatever, get something cheap and safe that gets you from a-b.
BTW all of these are your business plan to present to your folks, essentially you’re taking a risk and you’re presenting you’ve done your diligence and research and have though about alternatives, mitigations, plan b. People you know will then judge whether to invest in your plan (even if it sounds perfect), based on their understand of your own ability to stick to plans, and will do their own risk mitigation.