Can you implement solely the transactional enhanced eCommerce without the product impressions etc.
The documentation indicates to me that you can use just pieces of it. It says:
There are multiple types of ecommerce data you can send using analytics.js: impression data, product data, promotion data, and action data.
It doesn't say you have to send any of them, it says you can. Furthermore it says:
Combining Impressions and Actions
In cases where you have both product impressions and an action, it is possible to combine and measure this in a single hit.
This further implies that sending impression data is optional. It only combines the data if it has it to do so.