Västerås is located about 100 km west of Stockholm. Stockholm-Arlanda Airport is situated right in the heart of Scandinavia, with most of Europe only three hours away. The airport is a key hub both for flights within Europe as well as destinations all around the world, thus perfect to use for chartered flights.
Transportation from Arlanda Airport to Västerås takes approximately 1 hour by car/taxi and 1.5 hours by bus. The price for a taxi is approximately 1700 SEK. The recommended taxi companies are TaxiKurir and Sverige Taxi. You can book at: https://boka.taxikurir.se/en/ and https://boka.sverigetaxi.se/en/
VY bus4you (vybuss.com) offers direct, very frequent, and comfortable connections between Arlanda Airport and Västerås. The price for a one-way ticket is approximately 220 SEK. The ticket cannot be bought on the bus; you must use either the purple ticket machines (they accept debit/credit cards) at the airport or book them online (vybuss.com).
An alternative way of transportation from Arlanda Airport is to take the Arlanda Express train (https://www.arlandaexpress.com/, approx. 20 minutes) to Stockholm and then a train (www.sj.se/en, approx. 1 hour) from Stockholm Central Station to Västerås.
Other airport options in Stockholm include Bromma and Stockholm-Skavsta; the latter is very well connected through Ryanair and other low-cost airlines. Bromma is nearby the Sundbyberg train station, where it is possible to board the train from Stockholm to Västerås (www.sj.se/en). From Stockholm-Skavsta Airport, there are buses going to Stockholm Central Station, and from there you can take the train to Västerås.
Finally, there is the Stockholm-Västerås airport, very close to the Västerås city center (less than 10 minutes by car). This airport has connections to, e.g., London (Stansted).
Please note that Sweden is mostly a cash-free country and payment by debit/credit card is the most convenient and accepted way to pay in Sweden.
You can find additional info on how to travel to Sweden (including VISA) here.