Personally I think - if you want to - go ahead and set up a part time business, but invest the 20k in something else... a small part time business should be set up using as little money as possible. I've a (seriously) part time service business that cost less than €100 to set up, but should generate income (after costs) of 4k this year and 8k or more next year.
I believe you will be tempted to spend more than you actually need to, simply by the fact that you are mentally assigning the SSIA money to the business. Sit down, think hard, make a list of every conceivable cost, then cost the first years operation. Think about the revenue, will it arrive after or before major costs have been experienced? If you think the business will cost €10k in year one, add a third to costs for worst case scenario and reduce revenue by a third. Will you need 10k cash from outset or will the revenue cover some of the costs across the year. You may only need 5k to start.
Once you've figured out the amount you really need, say 5k, put 10k in some investment, 5k in an esay to access account (e.g. a deposit account) and the remaining 5k into the business. That means should the 5k be insufficent you can withdraw another 5k easily to support the business, yet the remaining 10k can continue to grow somewhere else.
Best of look if you decide to take the plunge.