Re: First time boat buyer: What's the boat equivalent to a 10 year old Honda Accord?
Good analogy - but it doesn't translate quite as well with boats: weight and balance and efficient use of space are completely different (sorta' like with airplanes vs. cars). However, the basics of your question are: I want to start out cheap and reliable, pretty is not a major concern. And that is (IMHO) the best way to start out boating.
Budget set at $8K - first hurdle covered. And, BTW that's plenty to get a good boat. (I've actually never spent that much on a boat before

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Most 18' - 22' boats are rated to hold 8 people, BUT ... it'll be snug with that many on board (especially when you add water toys, coolers, towels, etc., etc.) If you're thinking your family plus taking friends out, consider how often you'll do that and that'll tell you how critical the "snugness" is.
Personal opinion - once you get bigger than about 24', it's a much steeper learning curve for a new boater. Not that one can't do it, but I think it's harder on the learner. I also believe that it's easier to learn on an outboard, but there won't be many of those in 18' - 22' - and it's not that big of a deal.
Brand means nothing (or very little) in used boats. Care and condition are key. Aluminum boats will likely be in better (structural) condition and require less care, but fiberglass boats are much more plentiful and (well cared for) last a long time too. I've always owned fiberglass, myself. For I/O's, keep yourself in newer Mercruiser or Volvo units. There are a couple of drives that are now obsolete and add a level of difficulty (see the one in my signature

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BTW - Welcome aboard!