Beware, my friend, beware and repent!
It is still time to renounce and abstain!
Another nearly lost soul begging for guidance and help facing the temptations of the vile steamal sin but you, forces of darkness unleashed, recommend a Märklin 4097!
Which is good piece of advice, imho!
The real question, however, is, if you want to buy a steam engine of recent production date (Wilesco, Mamod or Jensen... = grey sin)
or if you would like to venture out into historic steam engines (Märklin, Doll, Schoenner... = pitch-dark tarry sin).
You can find smaller and simpler varieties of fixed cylinder engines (which are not necessarily double-acting...) by Bing or Falk or Falk-Bing which come relatively cheap...