My dad is looking to buy a skid steer. He really prefers the JCB brand because of their side entry capability. Technically he's looking at a compact track loader from JCB. Seems the 270T is the largest without DPF's and DEF. The primary use of the machine will be ranch work - brush clearing, cedar management, mesquite management, road grading, etc.

Is the 270T big enough for this application?

What attachments are must haves and what brand?

TIA

A good grapple.
I have the aforementioned tree puller. It is really a good piece of equipment. You will find that rear counterweights on your machine will make it work much better.
The tree puller puts a lot of force on your arms and pivot points. Especially if you pull larger trees. Keep those joints well greased daily.
A forestry service guy up here a few years back used a mono boom skid steer and it flexed enough using the tree puller that it rubbed the cab.

I wouldn't try to use a single arm telescopic boom like the JCB for grubbing brush unless it is very small brush. It's perfect for loading stuff on to trailers though. A cousin of mine has two of them for his sod farm and absolutely loves them for loading trucks.

If main objective is clearing land with a grubber and rake I would definitely go with dual armed boom.

Other attachments to consider would be a dozer blade, grapple, post driver like a Danuser or Montana and pallet forks.

I have a Cat 299D3 with hi-flow hydraulics, and have a grubber, brush rake, grapple, hydraulic breaker, cactus forks, smooth edge material bucket, pallet forks, shredder and round bale spikes. Working on getting a post driver, MB Crusher Bucket, FAE rock mill, 6 way dozer blade and trencher. Wanting to start a business and get out of the oilfield some time in the near future. I feel like I could stay busy year-round with those attachments.

yea im not sure id want to use a stump/tree puller on a machine with just one loader arm to the side like that.

The guy up here that had one was also very leery of lifting anything real high with any weight with his.

I have a small and heavy grapple, Danhuser puller, forks, bale spear, tree terminator shear, 6 way bulldozer blade, toothed bucket, smooth edge bucket and a mulcher/shreader.
One of the coolest attachments I rented was a trencher and a cement mixer.
On the trencher it is really advantageous if your machine has an auto creep and tracking adjustment so you can keep the ditch going straight.
The other input I can give is on the six way blade. Skid steers are so short coupled that it is really hard to spread gravel or dirt level. Any up or down in your ground causes ridges and valleys.
It's great for roughing in work, but check out the floating edge type with the gauge wheels for doing nice smooth road work.
Also make sure you get a wide enough 6 way that when you go to max angle that your wind row doesn't end up right where your tracks or wheels are. Lots of folks buy the blade the width of the machine and you actually need much wider when you go to max left or right angle.

Well here's the thing

My dad can't get into a skid loader unless it's a JCB side entry. He is "mobility restricted" although he refuses to admit it. He's actually bound to a walker and yes probably has zero business in a skid loader but as he says - if i find him dead out there someday, he died doing what he wants. But then I struggle with "what if he's suffering for days" so I have automatic alerts on his life 360 to make sure he's still coming and going to his house OR I have spoken to him regularly.

His day to day piddling is on a can am defender 6x6. He manages to spray mesquites from it.

That changes the dynamics considerably sir. I've been there. My dad was the same way.
More power to him and you for letting him continue to want to do things. That way he can still have something to look forward to and feel like he is useful and accomplishing something. That may make just enough difference to make him still look forward to life and getting up every morning rather than just quitting and getting sedentary.
Absolutely go let him get it and work in it. It will help keep his mind sharp and make him happy to boot.
If my dad was still alive, and by having an easy way of getting in it was by a certain design so he could….I would be buying it too!!