Regarding Purchase of Ipad

Connection of external storage devices is possible - with some caveats...

To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT or APFS with a single partition. As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required.

If your iPad Pro has a Lightning port, you will need to use an Apple Lightning to USB3 Camera Adapter to directly connect USB devices to the iPad. This adapter has both a USB 3.0 Type-A connector and an additional Lightning port to supply power, from your charger, to both the iPad and connected device. The external charger must be rated at 12W or more.

If you have one of the newest iPad Pro models, with a USB-C connector, you may need to utilise a powered USB hub (i.e., it has its own source of power) - connecting both the iPad Pro and the storage device to ports on the hub.

Some additional information can be found here:

Connect external devices or servers with Files on iPhone - Apple Support

https://www.imore.com/how-use-external-hard-drives-your-iphone-or-ipad

Whilst external storage can be useful for archiving data, it is not a substitute for internal. In many cases, files and media can only be accessed from local storage - such that the material must be copied to the iPad before Apps can access or manipulate the files/data. An iPad is not expose the filesystem in the same as you may be familiar when using PC, Mac or other computers.

You can directly compare iPad models here:

iPad - Compare Models - Apple

In summary, iPad8 is the entry level (cheapest) model in the current range. At the other end of the scale are the iPad Pro models - representing the current state-of-the-art with highest performance (and greatest cost) and most features.

Between these two extremes you will find the iPad Air4. This is a high-performance iPad, but lacks the largest screens and some features. If you are looking for a small screen device, you also have the iPad mini4 - its performance being closest to that of the larger iPad8.

As for which iPad is best for you, only you can decide.

“Best” really is subjective. You need to understand what you intend to use the iPad for - and extent of your budget.

Simply choosing a current model of iPad Pro at least guarantees that you will have an iPad capable of anything that iPad can do - at the highest levels of achievable performance. If you are uncertain - and have the budget - an iPad Pro will serve you well.

Regardless of your eventual choice, carefully consider the economy of selecting a WiFi Only model of any iPad. Only the WiFi+Cellular models include GPS functionality - the WiFi Only models do not. 

We frequently see complaints and dissatisfaction from users who subsequently learn of this limitation after purchase; not only does this fundamental difference affect the reliability and accuracy of Location Services, it can also influence use of some Apps - such as mapping. Selecting a WiFi+Cellular model of iPad doesn’t lock you in to using Cellular connectivity, but in addition to providing GPS functions, does provide the flexibility of using Cellular connectivity if your needs require this functionality.

Whatever model you choose, you would be well advised to purchase as much internal storage as you can afford - as internal storage cannot be changed, upgraded or extended. The available internal storage is fixed for the entire life of the device. If you run out of internal storage for your documents, photos and other data, you will forever struggle. Better to have more storage than you need, than to need more storage than you have.

A good rule of thumb is to quantify how much data storage you will need - add a healthy margin - then double it. If you are unable to quantify your data storage needs, it becomes more difficult; whilst I cannot make definitive recommendations, personally I would council against purchase of any iPad with any less than 256GB storage - 512GB offering a greater level of safety. 

Within the Apple Support Communities we see regular tales of woe and disappointment when storage requirements are exceeded; by contrast, we never see complaint of having too much available storage.

Tell us what requirements you have. What you plan on doing with it. What kind of budget you have for it.

Provide some details, and someone may be able to advise.