I have spent months doing a LOT of research into this same scenario for us. We got our place 5 years ago but were only able to install gas this year, due to a moratorium on cutting the street to access the gas line on the other side.
Let me address several points.
1. You need to do a good heat loss analysis, to know how to properly size a gas boiler.
- A good app can be gotten from slantfin.
- The "old way" (very conservative) was square footage * .35. Whatever this gives you is oversized but at least gives you a quick reference. .25 is probably a much better multiplier. But - do the job right and do a full heat loss analysis.
- An oversized boiler is not a good thing. It'll cycle on/off too often, which shortens its life. And it'll use too much energy = $ spent.
- So please take careful measurements and don't be too surprised when you really only need a 50K or 65K or 75K boiler. Most people have boilers that are way too large - especially if they are oil boilers. They really don't make "small oil boilers". They start pretty close to 100K btu.
2. While you are at it ... figure out what can be done to help your house envelope. We did a bunch of air sealing and insulation (our latest place is a victorian) and saved $700 this year in heating oil - and this was a brutal winter. You can get big gains from sealing every penetration to the attic (use fire rated foam), and adding more attic insulation.
3. Some oil boilers CAN be converted. But ... it may not make sense to do it, even if it's possible.
- Many oil boilers are oversized for their needs. A conversion gas burner can't be "sized right for the house" if it is "too small for the boiler". Here's an example. Let's say you have a 165K oil boiler. Let's say the real heat loss for your house is 70K. So, a 80K boiler is probably about perfect.
BUT ... you CANT install an 80K burner in a boiler designed for 165K. Sure it'll fit BUT it's just not big enough for the design and you can have nasty issues in your flue and boiler chamber, even leaking gaskets, etc.
You CAN downsize say 10-20%. But .. you can probably get an 85% efficiency brand new gas boiler (i.e. Burnham ESC series) for close to the cost of a conversion burner.
IF you are REALLY lucky, your oil boiler is only a 3-4 section. If YES and it is relatively new, then MAYBE it makes sense for a conversion boiler. BUT ... your chimney would also need to be stainless lined (state code dependent) to allow this.
Anyway ... what I'm really saying is PROBABLY a conversion gas burner for your oil boiler isn't going to make sense. But if your oil boiler is SMALL then MAYBE ...
4. High Efficiency Gas Boiler vs "Standard" Gas boiler. You CAN buy an 85% efficiency Gas Boiler for only about $2100-2400. These also qualify for the $300 rebate (ends very soon). Several companies make these. Burnham is one of these companies - their ESC model qualifies. Think of this as good old cast iron with smarter electronics upgrades. It'll probably last 25-30 years. You can even add an outdoor reset to these.
Ok so to me this is really a numbers game. A "high efficiency" wall mounted boiler will cost probably at least $1000 more than an 85% efficiency model. They use aluminum or stainless steel for their chamber. It'll also probably only last about 15 years. It will also be more touchy and require more skilled annual maintenance than a good solid cast iron boiler.
So .. any savings between 85% and say 95% with natural gas will be eaten up by annual maintenance and the high efficiency unit will probably be replaced in probably 15 years.
So ... it's just not worth it - in my opinion - to do a high efficiency modulating condensing boiler.
Note: you MAY need to add one or more pumps to your system. You mentioned it is a gravity system, which usually means it is a great big system with big pipes that originally was steam or water - and perhaps coal-fired, back in the day.
5. No matter what you decide. It is IMPORTANT that you have a good installer, and that he designs and builds "boiler protection" into the design - to ensure that cold return water is pre-heated to about 130 (or X degrees) before it gets back to the boiler (different boilers have different tolerances on return water temp).
Think of this as a loop near the boiler with a thermostatic valve to ensure that return water is mixed (when needed) with hot water to keep it at health levels.
If cold water is allowed to come into the boiler, then you'll have a boiler that develops leaky seals in probably 2-3 years. And you'll think that the boiler is at fault instead of your installation. This is a more common problem today - since boilers are more often right-sized than they used to be (or not massively oversized). Most old boilers were so powerful that they got by without this type of protection. Example - my current oil boiler is sized for 240,000 BTU. How crazy is that? I downsized the nozzle but even still it is 165,000 BTU!
6. You may wish to consider whether you wish to have a separate gas water heater OR to have an indirect water heater that is powered by the gas boiler.
Burnham makes nice indirect units that are stone lined. Think of this as one additional circuit that takes PRIORITY when it needs it. You do NOT need to change your boiler sizing if you add an indrect water heater.
A separate gas water heater needs it's own 4" vent. But ... if it goes, you just replace it. If your boiler has an issue you will still have hot water.
An indirect 40 or 50 will give you basically unlimited hot water, for most people's needs (unless you have a big hot tub? and someone else wants to take a shower with jets). It'll also last probably 25-30 years. But ... you will need to run your gas boiler all year round. It also won't need to be vented
OK - well I could go on and on but this should give you something to chew on. If I were in your shoes (and it so happens that I am), I'd just get a solid cast iron 85% gas boiler like the Burnham ESC3 or ESC4, get a GOOD plumber who knows how to design boiler protection, and consider if it's better to have a separate 40/50 water heater or an indirect water heater.
I hope this helps and wish you the best.