Don't be afraid of a few hours on a used tractor... bought mine with around 300 hrs on it and remaining warranty. Absolutely no problem...and buy in cost was less at the time...resale prices in your area may vary. All else being equal, local dealer and expertise is very useful.... have you looked around for a used, low hour, 4wd, loader JD tractor??
Yep, tough to move to different tractor from what you have..more $$ hurts.. perhaps you could borrow a 4wd/loader tractor from someone local for a short time to get personal experience on your place.
Fundamentally, tractor size and implements is realistically determined by the maximum you really need to do with a tractor (pulling power, FEL lift weight, etc) not the average/typical thing. If you target the average/typical, then much of the time you are fighting to get stuff done because you are underpowered or do not have the right attachment, or that extra hydraulic port, etc....
I fought thru several tractors (2wd, less hp than needed, no loader) and MADE them work, beat my body up, just didn't do some things, etc .... It is absolutely amazing how many more things I get done with MUCH less tear on my body now. Fundamentally, I moved from 35hp to 50 plus 4wd plus loader and matching equipment.
With 40 acres and part time, you will be astounded how diverse the tasks are you will seek to do....always more to do than you have time for...your tractor and attachments is the only time and work multiplier you can obtain. The pages of TBN are filled with the benefits of 4wd and usefulness of a loader. If you could step ahead 5 to 10 years and look back over the accomplishments with your current tractor versus a 4wd/loader tractor, the difference would be quite obvious. My boys frequently tell me, and rightly so, that I should have gotten my current tractor years and years ago.
So, all said and done, it's your $$ and only you know what else is tugging at your funds.....but if at all you can swing it, I'd hunt for a way to start out on your 40 acres with a tractor that has more power, function and features than you now think you need...you will DEFINITELY find ways to use all that and find a need for more.... almost everybody on TBN has had this experience, although some would argue the point.
You purchased the land to do things with it and shape it to match your dreams....the single tool most capable of making your vision come thru is your tractor and implements....matched with your vision and skill.
I wish you all the best, and continued contributions to TBN:thumbsup: