How do I know whether my passion will make a good business?
Good. We all do. Problem is, just because we get to do what we like and get paid for it, doesn't mean we won't have the headaches of figuring out what every other entrepreneur has had to: how to run a business.
You sound like you need to really make a business plan before you decide to open a shop. By doing that plan, you'll see exactly whether or not it's a good idea to open such a specialized business, and whether or not it's a good idea to compete right next door to your competitor. Hint: NOT a good idea.
There are a number of ways to overcome the threat of competition. If the existing shop has a department that does repair, it may be simply because they have to to satisfy customers. In which case, a possible contract agreement may be a solution. That's an opportunity. You could take their repairs and some headache off of someone's shoulders and there's a sweet deal. Referrals and specialized business to go. It is somewhat rare that those kinds of deals happen.
You're missing a lot of info Sammy, who is your advisory team? (legal, financial, business, mentor) these people are necessary to keep different eyes looking out for your best interests. If you have none, there are ways to find good advice on those subjects. Go look for some, you'll thank them for their help later.
In marketing, you need to do a S.W.O.T. analysis. (strengths, weaknesses, opportunities, threats). This will tell you whether or not it is a good idea to be so focused or to compete right next door to a similar shop.
It has happened but most times businesses get a "non-compete" clause in their leases that keep what you're proposing from happening. Again, if they are open to contracting you for repairs so they can concentrate on sales, you're in good. If not, it may be a better idea to find another location and plan out your marketing efforts accordingly.