Hi, Mike! Welcome to the club!
New member here, ~140 hour VFR only PPL, average skills (at best). I did most of my primary in a 152 and every time I get back in one I really want to pull the trigger. I am running some numbers and would love to hear your thoughts on the following. I hope it is not rude to sort of just show up and ask a million questions, but I really do appreciate any advice!
I'm in Denver (BJC), but with no illusions about flying a 150/152 into the mountains. My experience is in a 152. With that said, would DA be significantly more of an issue in a 150? The "mission" is me just flying around and occasionally my wife and I (~340# together) going on short hops together.
I'm over by Erie, based out of Parkland.
Even though 150s have less HP than 152s, they're usually a little lighter. Older model 150s tend to be lighter and have better useful load than the later models. I learned in 152s over in Boulder and have owned a 150. If you can handle a 152 up here, you'll do just fine with a 150.
Is budgeting $100/month for a tiedown reasonable? Is an uncovered tiedown a terrible idea?
Since I'm at a private airpark and don't pay tiedown fees, I really can't say. You'd have to call around to the local airports. The good news is that you have a few choices. Both Boulder and Erie are within easy driving of KBJC. In this area, a tiedown is not unreasonable. We're a semi-arid environment so moisture is usually not much of a problem. Big issues around here can be wind and the occasional risk of hail. My plane was tied out for most of it's life. It's only been hangared for the last 6 years.
The best thing to do is to fly often. Airplanes that sit tend to rot.
When my plane was tied out, I'd go and remove any snow or ice as soon as I could after a snow then fly the plane.
Is budgeting $1,200/year for insurance reasonable?
I pay around $500/year for insurance on my 150. It hasn't changed much since I bought the plane. When I bought the plane, I had less than 100 hours.
Is budgeting $1,000/year for the annual and $1,500/year for unplanned maintenance reasonable?
Annual inspections can vary widely, depending upon where you get the work done and the condition of the airplane. I use a freelance mechanic (A&P/IA) and my annuals are pretty inexpensive. I also don't let maintenance issues go until annual time so there usually are no surprises. There are some good threads discussing annual inspections and people's thoughts on them in the Tech section. As a starting figure, your numbers are probably reasonable.
Is budgeting $500 for a pre-buy inspection reasonable?
Again, you'd have to call around. I would think that would be a good place to start. That's the base rate a few A&P/IAs I know quote for an annual on a 150 so I would think you would be able to get a pre-buy for around that figure.
How do you recommend thinking about an engine overhaul reserve? My first thought was the overhaul cost divided by (TBO minus SMOH) but I wonder if that is overly conservative.
When it comes to things like major overhauls, I don't think you can be too conservative.

Good luck in your search! Keep us posted.