It has nothing to do with "size issues". I have a 61" ferris, a 48" quik trac JD, and we are going to have a 36" walk behind on this trailer. Plus the front 2 feet of the trailer will be taken up by the storage box. Also, I will not be using this trailer just for mowing, we do landscaping and need the capabilities to haul a bobcat or utility tractor. I do not want two trailers if i don't need two trailers, and if i can add heavy axles to this trailer, i won't NEED two trailers. It's about only investing in one trailer that can do multiple things.
My truck was about $20,000 when i bought it 6 years ago. I don't know where you are getting this $45,000 number from.
Two 3500# axles might work for you, but I do not want to be limited by my trailer.
Oh well, let me expound.
If you're running a landscaping outfit with it by all means get the right size. But I didn't see that in your post. You mentioned the mowing setup you used and I pointed out that 3500's would be fine and the length was adequate. I have done this a while and know your mowers would require about 15.5 feet of trailer length with a side gate (32" mounted sideways to roll off the side ramp).
Your mowers and gear plus a couple of trim mowers etc weigh around 3,000 lbs. With the trailer weight that's about 4,500 lbs tops. So I assume 7,000 lbs of axles would do. But bigger axles isn't what I meant, as they have little penalty.
Length (and actually width too, and eventually weight) is the real issue. I see crews hopping curbs, backing down streets they can't turn around in, running accross the grass because they can't fit in a tight driveway, blocking multiple driveways, taking out signs, all because they have a huge trailer and truck combo with only the typical ZTR and wb on board that even a single axle trailer towed by a Ranger could handle. If they want to stop for a meal they have to find a place with 10 open spaces during lunch rush or circle the block while someone runs in to get the food.
Trailers are about the cheapest thing we buy per year of use. Initial cost doesn't even matter much. But a longer trailer is simply harder to maneuver sometimes. Toss another 4' onto the rear of your rig and there will be times you will regret it eventually.
$45,000 is what some spend on a NEW diesel crew cab type truck with some options on it. If you got a $20K diesel it probably wasn't new. I'm just saying "size" can lead to a cascade of expenses. Whether it's going from $15K to $20K on a used model or $35k to $45k on a new one, it costs more to haul more, all other things considered. 4500 lbs of mowers/machines/trailer is something even compact trucks can handle ok, and a light duty full size can do with ease. Keep stacking on weight and size and braking capacity needed and yes, you can need to go up a class in truck.
This time last year you could drive a new Ram 1500 quad cab Hemi express (20" wheels, tow pkg, power stuff, etc) off the lot for $25,000. I know I'd rather have a new light duty truck with 5 years of powertrain coverage than an older high mileage HD diesel if it otherwise met my needs. The ride and noise factor alone would tip the deal.
My HD chevy transmission died just under warranty and would have been an $8,000 repair a month later. My new F150 hauls fine and drives a ton better. If you need to haul a big tractor or heavy materials, go for the gusto. Just realize there are trade-offs.
When I say "size issues" I was speaking of the advice given. It's typically "bigger" here no matter what the question is. I'm mentally picturing the ridiculous setups I see all the time out there, where a 20' monstrosity built to carry construction equipment, towed by a 3500 dually, has got a couple of 600 lb gear drive wb's and 2 21"'s on it, struggling to get 10mpg. All because once a year they use it to haul a couple pallets of sod at once instead of just one. Here, for $50 delivery fee a supplier brings sod to your door from 2.5 hours away, free if you order a half truckload or more. So I skip the jiggly bumpy ride.
Do lawn maintenance day-in-day-out and the little conveniences matter. Not having to sweat tight turns in traffic, or being able to get gas at the same pump your mowers get fuel from. Or not having to wait for the one diesel pump in yuppietown to clear. Or not having your rig parked on a busy street at the crest of a hill instead of a safe driveway because it's too long to fit. Hell, even being able to fit in a drive thru when it's 100 degrees out and you are in a hurry. Keeping size/weight down is part of that.
Then again, maybe everyone here on lawnsite lives in Montana and has a huge turnaround at their shop and at every jobsite.