I think it is definitely wise to consider the minus points, who will dog sit when you're on holiday (or kennel costs), possible allergy problems, whether you are upto training a dog (previous experience etc) and whether you can devote the time to training them from puppyhood (or if you get a rescue, devote to habilitating them)
i generally have to persuade my dog she wants a walk when it rains.
my food doesn't cost much (www.anglianmeatproducts.co.uk - much cheaper than buying the same stuff via Pets at home - plus a big bag of biscuit once a month) for a small dog not a great cost
grooming cost/ time - my dog only sheds into her coat so no hoovering but i bath her once a month and do her hair (maybe 3 hours? -would cost £40 with groomer about).
adult dogs can be left for longer periods but I think working a full day with one isn't really in order (i'd planned to give up work, it didn't happen, i ended up with a very stressed dog - even for three days per wk)
Getting a dog from a good breeder with money-back guarantee with all health checks from parents can reduce the riskof getting unhealthy pup - though even the healthiest dog can throw you the occasional bill for e.g. broken bones! (x-ray, painkillers etc)
My dog hates other dogs so i have to be very careful with her (means i actually avoid most other people with dogs, normally they would help you to meet new people)
Even a small dog makes your house safer (the barkng) and more of a home.
for a small dog a dog=door can make it easy for them to go out during the night/ day when they need to go.
i think they can be a great addition to a family, but think as much as you can about breed - and expect to do all the work yourself (as already said, kids lose interest)