If I buy more space for my first generation iPad will it update from ios5.1.1 to the current update?

No.

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And you cannot 'buy more space'. The storage on iPads is fixed and cannot be increased.

Buying iCloud storage space DOES NOT increase storage on an iDevice.

You must purchase a new or "newer" (used) iDevice with larger storage capacity.

The orginal 1st generation iPad cannot support an IOS version any higher than IOS 5.1.1.
The hardware in the iPad 1 is NOT capable of supporting newer IOS versions.

iOS 5.1.1 is the highest iOS the original iPad can support.

No way to change this fact.
Sorry.
The original iPad is nearing the 6 year mark (April 2016), it has not received any updates since 2012.
You may want to consider upgrading to a new or "newer" ( used ) iPad model.
To insure some future proofing, I would recommend looking for 1st Gen. iPad Air models or greater.
iPad 2 and 3 models are having some issues with the most current iOS versions now available.
And many current iOS features are not supported on iPad 2, 3 and 4 models, either.

Its not just a memory capacity issue. The 1st gen. iPad only had 256MB of RAM and much lower processor and GPU than current iPads. Even if you could install the latest iOS9 on it the performance would be terrible.

That iPad has had a good long life in technology terms but it really is time to consider a hardware upgrade. 🙂

And you can ignore me as MichelPM already said all this. 😝